This manual explains the installation and operation of the ZERON 550 rally computer. Operating procedures for many different rally situations have been included, but obviously it is not possible to cover all of those you may encounter. A thorough knowledge of how your computer functions will help you adapt these instructions to local customs.

It is highly recommended that you read this manual carefully and then practice with the computer before you compete on a rally. An old rally with mileages and elapsed times can check your mastery of the 550 without the added pressure of staying on time.



The 550 utilizes one of the newer integrated circuit technologies that, unfortunately, is susceptible to damage by static electricity, e.g., the kind that shocks you after walking across a rug on a day when the humidity is low. Therefore, several precautionary steps should be followed when you handle the 550. When the computer is in its case there is no danger of harming any of the electronic components. You should, however, avoid touching the contacts on the rear connector. The same applies to the readout connector. If you must remove the computer from its case, hold it by the front panel. Try to discharge yourself by touching a grounded metal object before you start. Once the computer is out of the case, avoid contact with the printed circuit boards and the components mounted on them. Don't worry if you accidentally touch one of the prohibited areas. Most likely no harm was done.

Mounting studs are located on the top and rear of the case. Additional holes are drilled in the bottom of the case to attach hardware from underneath when needed. Refer to the illustration at the end of the manual if you find it absolutely necessary to drill more holes. In general, you may add holes at any location on the top or bottom of the case. There are also 2 small areas on the back where holes may be drilled. Do not use the remaining part of the back nor the side panels. The circuits are at minimum clearance opposite these surfaces.

Remove the computer by sliding it forward after the 4 screws in the corners of the front panel have been removed. You will find a short cable running from the main chassis back to the heatsink mounted on the back of the case. Beware of nicking the cable as you work. When you re-assemble the unit, loop the cable above the printed circuit board it's wired into, to prevent it from being crimped between the box and the case.

The heatsink is connected to the +12 volts side of the car's electrical system. Be positive that it can not touch the car's chassis. It won't hurt the computer but it will blow the fuse and shut you down!

The heatsink, as you might imagine, gets hot. Not hot enough to harm your car but it will surprise you if you reach around behind the computer and touch it. For you navigators with drivers who can't keep their hands off the equipment, you may give them blanket permission to touch it at any time.

The 550 is built with components that operate at temperatures between -40 and +85 degrees centigrade. Operation outside this range may cause the computer to calculate incorrectly although no permanent damage will occur. While you won't rally at the low end, temperatures at the high end are possible under certain conditions. On hot days when the car is closed and in direct sunlight, temperatures may exceed the upper limit on the dash board. Watch out for this at rest stops, lunch, and other times when you will be out of the car. Park in the shade or so the dash is not in the sun. It's also advisable to turn the counter displays off. This has several advantages. First, there is little danger of the computer overheating if it's left in a poor location. Second, the computer draws only 0.06 amp (instead of 1.5 amps) so it can't run down a weak battery. Finally, your competition won't be able to check their mileage and computed time by comparing them with yours.


Mount the readout in front of the driver so that it can be easily read at a glance. Once you have removed the readout's back cover, the proper locations for possible mounting holes will become obvious. Apply the same precautions for the electronics and temperature that were mentioned above. A piece of white paper will keep the sun off the readout at breaks.

Sending Unit:

Disconnect the odometer cable from behind the speedometer, insert the sending unit, then reconnect the cable at the end of the sending unit. Make sure that the short square pin is in place to drive the speedometer. The cable should turn evenly with no binding. A small amount of speedometer needle bounce at low speeds is normal; large fluctuations indicate a binding cable. Check for any sharp bends you may have caused in the cable. It might be better to change the position of the cable or to rotate the sending unit on the back of the speedometer.

The sending unit can also be driven from a dead wheel cable or from an auxiliary cable connected thru a 1:1 tee to the odometer cable. In either of these two cases, static electricity could develop in the cable. Ground the sending unit with a wire between it and the car's chassis to keep this electrical noise out of the computer.

The sending unit is not weather proof so it may be damaged if left exposed. Always mount it inside the passenger compartment.

Standard Halda tee fittings are used at each end of the unit. If you change cars, these fittings can be replaced with a different type. It may then be necessary to adjust the washers located on the rotor. The bent washer applies a slight drag on the rotor. The drag prevents extraneous vibrations from causing false outputs. Increase or decrease the drag by bending the washer until the rotor offers a small resistance when turned. Check the drag every 10,000 miles if you leave the sending unit installed permanently.

Most British and French cars use odometer cables which terminate with a 0.118 inch square shaft. All other cars use a 0.106 inch shaft. Your sending unit is equipped with the proper rotor for your car. If you shift cars and need the other size rotor, they can be ordered from the price list at the end of the manual.

Electrical Connections:

Plug the readout directly into the connector on the rear of the computer. Snap the wire loop on that side of the socket into the cutout on the side of the plug. This locks the connectors together. The connectors for the sending unit and power are located on short lengths of cable wired into the plug. The pins are arranged so they can't be mated incorrectly.

Two pre-wired power connectors are included with your computer. The spare can be used in another car or for bench tests. Connect the red lead to +12 volts, the black to ground. A power loss, either +12 volts or ground will erase all of the computer's data. For this reason you should not select a power lead from under the dash. It may contain numerous slip-on connections as it is routed from point to point; your best tap point is at the fuse box where the main power lead enters. Attach the ground lead to any metal point that leads to the main part of the car's chassis. Using a cigarette lighter plug is inviting disaster.

Keep the sending unit and power connector pins free from dirt and grime. A light brushing with steel wool once a year will keep the contacts in good condition. Never wrap masking tape around the pins. The gum from the tape attracts dirt. You can use tape to hold the connectors together if you think they might separate because of the way they are located in your installation.

Notice that the power connector has 4 pins instead of just 2. The pins are wired in pairs, 2 for +12 volts and 2 for ground. If your installation requires additional connectors, follow this procedure of doubling up on the pins. This is another precaution against a momentary power interruption.

A 3 amp fuse is located in the line between the readout plug and the power plug. A replacement can be found at most service stations, electronics supply outlets, and TV repair stores. In desperate circumstances a fuse with a higher rating is OK, just remember to put the correct one in later.

Accidentally reversing the power leads will not harm the computer. It won't turn on. The fuse will blow if the sending unit was connected when the leads were reversed.

Two final items concerning your computer and the car's electrical system need mentioning. First, the computer will operate on voltages from 6 volts to 17 volts. Go below 6 volts and information starts dropping out. Your car's output voltage normally varies between 8 volts during starting and 15 volts when charging. A good battery is your best protection against the voltage falling below 6 volts.

Second, while the nominal output voltage is 12-14 volts, electrical noise and large voltage spikes are often present at the same time. Your computer has adequate protection to isolate it from noise sources (lights, fans, horn, etc.). The one exception is solid copper ignition wires. While few cars come equipped with these wires as standard equipment (Alfa Romeo's are one), if your car has them, they need replacement with standard carbon resistance wire. Don't forget the wire between the coil and distributor.

Porsche owners relax, The models with solid wires are also equipped with suppressers which eliminate the noise generated in the wires.

You may be tempted to "see what happens" before being convinced to switch the wires. In some cases the effect is obvious: the counters start rapid and sporadic counting as soon as the car is started. In other cases, hours of close observation may indicate that all is well. Don't be misled by this apparent lack of trouble. This type of noise is extremely elusive. Why risk getting bad checkpoint scores? Change the wires!!!


As you can see from an examination of the front, the 550 does not have many of the control switches found on other rally computers. At first glance this may appear to limit the versatility of your computer. However, as you continue with the manual you will discover how many of the operating functions have been combined to make the 550 nearly error proof yet still allow you to handle almost every rally situation.

Since most of the functions are interrelated, it is difficult to explain one without reference to others. Rest assured that when such instances occur they will be fully discussed in later sections. A sample rally is used to illustrate the steps you would take during an actual rally.


When the 550 is turned on, the displays will show a meaningless pattern of digits, and in some cases, blank digits. This random pattern will also warn you that the computer has lost power. If it occurs on your shakedown drives, a faulty power connection must be found and eliminated.

The odometer records official rally mileage from 00.00 to 99.99 in increments of 0.01 mile. Once the computer is corrected at the end of the odo leg, the odometer will match the rally's mileage. The count direction, up or down, is controlled by switches that you will read about later. To zero the odometer, press the small button by the left of the display while also pressing the button located just above the row of switches labeled CORRECTION FACTOR. The second button, which will be referred to as the "safety" from now on, has several purposes. One of them is to prevent you from accidentally zeroing the odometer. Because it must be pressed along with the other button, an unintentional bump on the zeroing button won't cause the loss of your mileage.

The auxiliary counter has three modes. The toggle switch at the left of the display selects the one that's needed. Mileage in hundredths of a mile is recorded when the switch is in the DIST (distance) position. This mode can be used for actions keyed to short mileages. For example, "Left 2.48 miles after the county line". The TIME mode is used for similar instructions. The "time" recorded is computed time, not real time. When the car isn't moving, the time isn't moving either.

The middle mode records the amount of a pause or gain as it is entered into the computer.

The procedure for entering pauses and gains will be discussed later. For now it's only important to know that they are recorded here in the middle counter. The toggle switch normally remains in the P/G position. After a pause or gain is entered, zero the counter so it's ready for the next action. You don't need the safety to zero. The auxiliary spends most of the time at 000; there's little chance of losing its data accidentally.

In place of a fourth digit, the auxiliary has a warning flasher to alert you when one of the rotary switches is not in its normal operating position--ON COURSE and PAUSE (+T). The counter, therefore, only counts to 9.99 where it starts over again at 000. Most time or distance related actions are keyed to short increments. For longer ones, the time or distance can be added directly to the readings in the odometer or computed time counter.

The lower display has two separate functions: A real-time-of-day counter, the clock, and a computed time of day counter (CTC). The toggle switch selects one or the other. The counters are completely independent, they share only the display. The clock registers while the CTC is displayed and computed time continues while the clock is viewed. The zero button resets both counters at the same time. It doesn't matter which one you are looking at, they both zero. The zero button is there only as a convenience for when the computer is first tuned on. Once the clock is set, there's no need to zero it again. If a time of 00.00 were needed in the CTC at some point in the rally, you would add or subtract the required number of pulses to the information already in it. The safety must be pushed to zero.

The displays' intensity is controlled by the switch located in the lower left hand corner. Use the BRIGHT position for daytimes rallies, the DIM for night ones. The OFF position turns the displays completely off.

Correction Factor:

The computer correction factor adjusts the sending unit input pulses to allow the 550 to run at official rally miles. The four digit factor is set in the switches labeled CORRECTION FACTOR. The numerals are horizontally oriented to distinguish then from the Speed 3 digits.

The formula for adjusting the factor at the end of the odo leg is:

                                  OFFICIAL MILES
                     NEW FACTOR = -------------- X OLD FACTOR
                                  COMPUTER MILES

Example: Your factor at the start of the odo leg is 4800. The odo leg is 14.92 miles long and your odometer reads 15.07.

                                 ----- X 4800 = NEW FACTOR = 4752

4752 is your factor for the remainder of the rally. Of course you can change the factor should conditions require it.

Before you run your first rally you may want to obtain a "base correction factor". This is the factor that runs the computer at statute miles in your car. Once obtained, make a habit of starting each rally with this factor. This normally starts you off within a few percent of the rally's mileage so it makes the odo leg easier to run.

To establish your BCF, it will be necessary to run the car against a known statute distance. Many highways have measured courses/mileage markers or use the odo leg of an old rally measured in statute miles. If a suitable course is not available, lay one out with the car's odo. A more precise BCF can be developed when the opportunity arises.

The course you select can have any length, however, it's a good idea to make it at least 5 miles. Set in a factor of 5000 and run the course. Use the formula to calculate your BCF. A rerun of the distance with the new factor will find your odometer reading dual to the course's "official" length.

Your exact statute mile factor does not need to be your BCF. Your calculations will be simpler if you round off the last two digits. As examples, 5529 would round to 5500, 3579 to 3600, and 4841 would go to 4800.

If you know the number of revolutions at the input to the sending unit, you can calculate your BCF from the formula below. As you can see, you must have a minimum of 480 revolutions per mile to run the computer. Most cars have between 800 and 1600. None are below 700.

                   CORRECTION FACTOR = --------------------
                                       Revolutions per mile

The 550 will also run in kilometers. The correction factor has been designed so that it would not normally go much above 6000. This gives you sufficient range to increase the factor to let the odometer read 1.00 at the end of 0.62 miles (1 kilometer = 0.62 mile). The 550 doesn't know what unit of measurement is being used, that's for you to decide. Speeds are now entered in KPH, not MPH.

Speed Banks:

The rally speeds are set in miles per hour with the switches located at the top of the computer. The correction factor corrects the entire computer, therefore, the speeds you enter are the ones given in the route instructions. You won't need corrected tables, conversion factors, etc. Any speed from 00.1 to 100.0 mph in increments of 0.1 mph can be run. A setting of 357 represents 35.7 mph. The speed is 100 mph when the switches are at 000, not zero mph.

The toggle switch between the two banks selects the active bank. The unused bank can be preset to the next rally speed. You may change the active bank's speed while the car is moving. This technique is helpful with small repetitive changes, say from 40 to 41 to 42... Don't use this with large speed changes. Unless you were particularly observant, you might momentarily have a very low speed in the bank as you change from one speed to another. Only 10 feet of travel at 1 mph is enough to get you a 5 or 6 at the next checkpoint. The best rule to follow would be to enter the unexpected or forgotten speed in the unused bank and then throw the switch.

Avoid extremely slow speeds whenever possible. This is one time when the 550's accuracy may hurt your score. Consider the following instructions taken from an actual rally.

34.45    33. "Stop" Change average speed to 1 mph.

34.82    34. "Go" Change average speed to 35 mph.

The time for this distance is 28.20 minutes. The rallyemaster was giving the crews a break at the gas station that occurred between the two instructions. When he measured the course, he rounded off the mileages to the nearest hundredth. Since he gave you two hard mileages, the time of 28.20 was correct. However, the 550 calculates on pulses received every 3-4 feet. If the real distance were 0.374 miles, the 550 would have calculated a time of 28.44 minutes. A .24 minute error at the next checkpoint. In this problem, you should have treated the distance as a transit zone of 28.20 minutes. No reasonable rallyemaster would expect you to maintain speeds this low. He will expect you to be able to calculate the elapsed time if he gave you the speed and distance.

Speed 3:

The 550 has a third set of speed switches at the bottom of the front panel. This speed bank is useful when the general or route instructions call for an automatic change to a given speed. By presetting this speed, you can change to it the moment it's required. The switch at the end of the correction factor activates the bank.

Checkpoint Switch:

This control locks the data displayed in the odo, clock, CTC, and readout whenever it is thrown to "checkpoint". The counters continue to function, but the new data is not shown until you return the switch to the off position. The clock and CTC are checked by switching between the two.

Throw the switch as you cross a checkpoint's timing line. The mileage to the line, your computed arrival time, the penalty you expect (on the readout), and your actual arrival time (clock) are all locked in the displays. If you are mistimed, the clock will verify the time in you want. After you have recorded any information you want, shut the switch off, the displays will immediately update to the counters' present data.

The navigator can use the checkpoint switch to hold information he wants to write down. Remember, the readout does not provide new data for the driver when the switch is thrown.


The readout displays the computed time error. This is the difference between clock and the CTC. As will be illustrated later in the sample rally, you will be rallying on a time-of-day basis. That is, when you are "on time", the time in the clock will be the same as the time in the CTC. When the times diverge, the readout shows the error.

Examples are:

                                 012 --   .12 minute ahead ("up")
                                 001 --   .01 minute ahead
                                 000 --    on time
                                 998 --   .02 minute late ("down")
                                 972 --   .28 minute late

From the above, you can see that the readout drops below 000 when you are late. You may think that a + or - indicator would make the driver's job easier. As you gain experience, you will learn that driving a little up is the most effective tactic for getting good scores. Since you will then spend much of the time up a hundredth or 2, a reading such as 002 will automatically indicate that you are ahead. It isn't necessary to associate an extra piece of data (the + or -) each time you check the readout. The numbers themselves tell you if you're up or down.

Mental arithmetic when you are way down is not required in most cases. Suppose you found yourself delayed by traffic and when you checked the readout it read 861. In a situation like this, it's more important to get back on time, not worry about being precisely 1.39 minutes late.

The following examples illustrate the relationships among the clock, CTC, and readout.

 CLOCK            CTC           READOUT
0000            0000            000
0122            0000            878
0122            0200            078
1234            1240            006
1234            2240            006

Notice how the readout in the last example appears to say you are up 0.06 minute when you are really up 10.06 minutes. You must always insure that the clock and CTC are in the same 10 minute segment. Though there is little chance of this happening while you rally on time, you should compare the counters at the end of the odo leg, the end of transit zones, and whenever you recover from being off course.

You will soon notice that the readout does not display all the clock or CTC pulses. If the car is stopped, the readout counts steadily downward as the clock counts up. Once you are at rally speed however, the readout will hold a constant value even though the clock and CTC are trying to pulse it up 1 count, then down 1 count, then up 1... The driver can hold the readout at a fixed number without the distraction of digits flipping up and down.

The above system is achieved by updating the clock and CTC once every 0.01 minute. Since the clock counts once per 0.01 minute, it appears to count in a normal sequence, e.g., 26.00-26.01-.02-.03-.04-.05 etc. The CTC behaves the same way when you are running at or below rally speed. Drive faster than rally speed and the CTC will receive more than 1 pulse per 0.01 minute. You may then see the CTC count from 26.00 to 26.01-.02-.04-.05-etc. Don't worry about the missing 3. It merely happened to arrive in the same 0.01 increment that the 4 did. When the counter updated at the end of the 0.01 minute, it showed you last pulse it had received.

The driver can also use the readout for pauses and gains. Suppose the route instructions call for a pause while the navigator is busy with a math problem he was given at the last checkpoint. The driver can run the pause on the readout until the navigator is free to enter it the normal way. If the pause were 0.25 minute, the driver slows the car until the readout drops to 975 (0.25 minute late). He then continues with 975 as his on-time reference instead of 000.

Mode Control Switch:

This rotary switch selects the operating mode for all the computer functions. The 550 is turned off when the switch is in the OFF position. All other positions are operating ones.

DIST ONLY (Distance Only) and D.O. REV (Distance Only Reverse) - These modes are used when computed time is not required, e.g., during an odo leg or transit zone. The odometer registers distance and so will the auxiliary distance mode. No computed time is added. As will be explained shortly, you are not prevented from adding or subtracting time in the CTC with the add time push buttons. This allows you to set the CTC to your time out from the end of the odo leg as you run it, or you could add a transit zone time to the CTC as you complete the distance for it.

ON COURSE - This is the normal operating mode for the 550. Both the odometer and CTC count forward. The warning flasher operates when the switch is not in this position.

OFF COURSE - The odometer and CTC are reversed when in this position. The readout is also reversed since it is following the CTC. Use the off course mode when you retrace the course after a course following error.

PARK - This position turns off the 550's calculating functions; use it when you want to leave the rally course during a break or pit stop. No mileage or computed time goes into the counters although the add pulse circuits still function. The blank positions on the switch are also PARK. If you accidentally overshoot one of the running positions, you won't turn the computer off or put it in test.

The park position also comes in handy when you are off course. As soon as you decide that you're in error, pick out a landmark (a tree, sign, etc.) and switch to park as you pass it. U-turn at the first opportunity and switch to OFF COURSE when you pass the landmark the second time. This technique eliminates having to guess at when to reverse the computer as you turn around. Switch directly to ON COURSE when you regain the rally route.

TEST - This position activates an internal pulse generator which runs the computer as if it were in a moving car. You can test or demonstrate the 550 in this mode. With a correction factor of 5870, the 550 behaves like it's in a car going 60 mph.

Pulser Switch:

You will utilize this switch, and the push buttons, when you want to pause or gain, add or subtract computed time, add or subtract mileage, and set the clock. The rotary switch selects where the pulses go, the push buttons the amount. For example:

Pause 2 minutes 30 seconds -- Set the selector to PAUSE (+T), press the 1.00 (100 pulses) button 2 times, the .10 (10 pulses) button 5 times. Remember, you must convert the seconds to hundredths.

The pulses go into the CTC and also register in the auxiliary, if it's in the P/G mode. The auxiliary double checks your pushes. If the auxiliary was in use with a distance turn, the pause would still go in, you cannot check yourself though.

Gains go in the same way. For a gain (or pause) over a distance, e.g., "Gain 1 minute in the next 1 mile", you can quickly enter the gain at the beginning of the distance, clear the auxiliary, then switch it to DIST to measure the 1 mile. Even when the car is moving when you start the gain, you won't miss more than 1 or 2 mileage pulses.

Since pauses and gains are simply the addition or subtraction of computed time, you can use these two positions when you want to set the CTC to a specific time. For example, you will set the CTC to your time out from the end of the odo leg. The CTC won't be counting, so use the push buttons to set it to the time you desire. Suppose the time is 48.00 minutes. Turn the selector switch to GAIN(-T) and, assuming the CTC was at 00.00, push the 1.00 button 12 times to subtract 12 minutes. At checkpoints you would use the same procedure to set the CTC to your assigned time out. The same is true of transit zones and time of day restarts.

+DIST and -DIST adjust your odometer. They are needed at the end of the odo leg to set the odometer to official rally mileage, to adjust for wheel slippage, and to correct the odometer at checkpoints where you have been fail-safed in after a trap.

Of course, all the procedures you have just read about can be applied whether or not the car is moving. It makes no difference to the computer.

The auxiliary counter does not register added or subtracted pulses in the time or distance mode. For example: a pause entered while the auxiliary was in use with a time turn would go into the CTC but not into the auxiliary. You would reduce the amount of time by the size of the pause. When you anticipate a pause or gain during a time turn, it's easier to base the turn on the time in the CTC. Suppose the CTC reads 34.91 minutes as you pass a clue that requires a turn 4.78 minutes later. Simple addition tells you the turn will occur when the CTC reads 39.69. The rallyemaster took into account any pauses and gains which occurred in the 4.78 minutes. Enter them as usual and turn when the CTC reads 39.69

Keep the rotary switch in the pause (+T) position except when you are using it when specifically called for by other requirements during the rally. Pauses are what occur most often between checkpoints. The flasher will remind you when the switch is in another position. Hopefully it will prevent you from putting the next "pause" into the odometer.

One last item will conclude this section. Pulses add or subtract according to how the rotary switch is set. The mode control switch does not affect the pulse direction. Even when the switch is in OFF COURSE, the pulses add or subtract as usual. If you go off course and enter a pause before you realize your error, you would enter a gain on the way back. A gain would be a pause as you return to the proper course.

Setting the Clock:

The clock is set like you enter a pause. The one extra step is to push the safety. The 1.00 and .10 buttons then add time to the clock. Press both the safety and .01 button to stop the clock. The clock runs at all times except when the two buttons are pushed. The following example illustrates how the clock is set to time of day.

It's now 24 minutes past the hour so let's set the clock to 25 minutes. Zero the clock and the CTC. Press the safety and then use the 1.00 button to pulse the time up to 24.xx minutes. The clock has been counting as you pulsed it so you can't set it to exactly 24 minutes. Next, use the .10 button to get the time up to 24.8x. When the clock's normal count cycle gets it to 24.99, press the .01 button. This will add one pulse and stop the clock at 25.00. When 25.00 minutes real time arrives, release the buttons. The first pulse (25.01) will occur 0.01 minutes later. This may all sound complicated but with a little practice it only takes about 20 seconds.

You'll learn that you must be fairly quick to stop the clock without one too many pulses in it. In the above example, 25.01, you may find it easier to press the .01 button, without the safety, a few moments before the time you want to stop the clock. When the desired time arrives, press the safety. The clock stops immediately without the extra pulse. That pulse went into the CTC. Since you will set the CTC later on during the odo leg, the 1 pulse doesn't matter. You could always subtract it.


Before you start, lets assume:

1. The first car leaves at 8:01.00 am.
2. Your car number is 10.
3. Your BCF is 4800.
4. The generals require an automatic speed change to 25 mph whenever the rally course becomes unpaved.

Install the computer and check to see that it's properly connected and in good working order before you leave home. As with all rally equipment, the 550 is useless if part of it is home in the closet when you're at the start.

Turn the computer on about 30 minutes before your time to leave the start. Zero the counters and set the clock to time of day. Enter your BCF and a "speed 3" of 25 mph. The remaining time can be spent to double check things in the car and to go over the generals one final time. (If you don't have a need for the "speed 3", put in 30 mph as a convenient number.)

0.00    1. Begin odometer correction leg. Leave this point at 8 am plus your car number in minutes. Take 30 minutes to reach instruction 8.

Check your odometer to see that it's zeroed, be sure you are in DIST ONLY, get your route instructions and leave. As you run the odo leg, set the CTC to your time out from the end of the odo leg (40.00). Enter the first two rally speeds.

8.93    8. End odometer correction leg at "Stop". Begin 30 mph.

Write down the mileage in the odometer (let's call it 8.82), adjust the odometer to 8.93, switch to ON COURSE and pull ahead to clear the area for other cars. The CTC begins counting up based on the first rally speed. The new correction factor isn't in yet but a short distance at the old one won't give you a timing error.

Calculate and enter the new factor(4860).

Up to now your readout has been of little value. However as the clock begins to catch up with the CTC, the readout will indicate the amount of time that remains before you are due to leave. Start the car up to rally speed as the readout nears 000. Be sure that the clock and CTC are in the same 10 minutes.


Throw the checkpoint switch as you cross the timing line. Compare the clock with the time in given you by the checkpoint workers. If there's a discrepancy, straighten it out before you release the switch. When you are satisfied with your time and have recorded whatever information you need, turn the checkpoint switch off and pull up to the out-marker. Adjust your odometer if necessary.

Most rallies use one of two common methods for starting you on the next leg. When the checkpoint zone is dead mileage, i.e., timing is from out-marker to checkpoint line, the procedure is effortless. At the out-marker press the CTC up to your assigned time out. Assuming that you have been given an adequate amount of time by the workers, the CTC will then have a greater time in it than the time in the clock. The readout indicates the time left before you start the next leg. Pull ahead slightly to clear the out-marker for other cars, then wait for the CTC to count down to 000 before you leave.

When the rally is timed from checkpoint line to checkpoint line (no dead mileage), the computed time between the timing line and out-marker must be added to your time out. For example, you are given a time out which is from the line, let's say 9:53.00, and you are now at the out-marker that is 0.10 mile away. Pulse the CTC to 53.00 plus the computed time from the line. In this case, if your speed was 40 mph the CTC would be set to 53.15. Again you would pull ahead to wait out your extra time. The manual's section on rally math gives you a method to calculate the additional time.

28.36             23. Begin transit zone of 20 minutes to next instruction.

28.90/0.00     24. "Max". End transit zone and zero your odo. Begin 35 mph.

At 28.36 miles switch to DIST ONLY and note the time in the CTC. During the transit zone add 20 minutes to the CTC. Plan to arrive at instruction 24 before your time is up. At "Max", set in the new rally speed, zero the odometer, switch back to ON COURSE and pull ahead to wait out the remainder of your time. Make sure the clock and CTC are in the same 10 minutes when you leave.


Some of the more frequently asked questions about the 550 are:

Q. How accurate is the 550?

A. Your computer is a digital instrument that will calculate a perfect time based on the speed and distance inputs. Checkpoint errors result from factors over which you have little control. For example, it is nearly impossible to precisely duplicate the path the rallyemaster took when he measured the course. If your mileage differed by 25 feet on a particular leg, the computed time would be off about 0.01 minute. When all factors are accounted for, you can expect to average about 0.01 minute error per checkpoint.

Q. How accurate is the clock?

A. The clock will stay within 0.01 minute during an all day rally. Of course you can adjust it if needed. The easiest method for doing this is to pulse up to the next even minute and start anew from there.

Q. Will shock or vibration harm the 550?

A. While obviously it's not made to be dropped or manhandled, the 550 can withstand all the normal jolts and bounces it will receive during a rally.

Q. Can I outrun the computer by driving too fast?

A. No. You could drive 100 mph with the computer set at .1 mph.

Q. Has anyone ever lost power during a rally?

A. Not when they followed the installation instructions. Several rallies are required before you gain confidence in your computer system. After that, you won't give it a second thought.

Q. What do I do if the computer breaks down?

A. Your 550 is built with high quality parts, however, there's always the possibility it may stop functioning properly. As a first step, you should write to Zeronics and describe the problem in detail. It may not be necessary to return the unit. Most of the important components are mounted in sockets so they can be easily removed for replacement.


The math problems you find most often on rallies involve the elements of time, speed, and distance. It is usually easier to work in minutes per mile factors (MPM) than with miles per hour. A factor table has been printed at the end of the manual for your convenience.

For those who are not familiar with MPM factors, a quick explanation is in order. Basically, a MPM factors tell how long (in minutes) it takes to run 1 mile at the assigned speed. The factor for 35 mph is 1.714. This means that it takes 1.714 minutes to run 1 mile at 35 mph. Moving the decimal point gives the time for 10 miles (17.14 minutes), 0.10 mile (0.17 minutes), and 0.01 mile (0.017 minutes--about 2 hundredths).

Notice how MPM factors would quickly solve the checkpoint problem described in the sample rally. In that case, you needed the time for 0.10 mile at 40 mph. A check of the factor for 40 mph (1.500) tells you it takes 0.15 minutes for that distance.

Missed speed changes can easily be "fudged" by using factors. To illustrate, suppose you forgot the speed change that was part of the last route instruction. You've been driving at 30 mph (2.000) instead of 35 mph (1.714). Unless you can risk returning to the execution point, you'll have to take an educated guess at the time required to correct the CTC. With the MPM factors you can quickly see that it takes about 0.29 minute less to run 1 mile at 35 mph than it does at 30 mph. For every mile you've been at the wrong speed you will have to gain 0.29 minutes. Let's say you estimate the distance back to the speed change as l.5 miles. The amount of the gain would then be 0.44 minute (1.5 X 0.29).

Two helpful formulas are:

                                       60                                ELAPSED MINUTES
          SPEED (miles per hour) = ----------               MPM FACTOR = ---------------
                                   MPM FACTOR                            ELAPSED MILES

Consider this problem: Change to a speed that will take you to route instruction 34 as if you had driven the first half of the distance at 30 mph and the second half at 40 mph.

Let's assume that instruction 34 is 2 miles away. We would then take 2.000 minutes for the first mile and l.500 minutes for the second mile. Thus for the 2 miles, it takes a total of 3.50 minutes. This is also equivalent to 1.75 minutes for each of the 2 miles; a MPM factor of 1.750. Using the formula, we can calculate a speed of 34.3 mph (60/1.75). The speed is independent of the distance used. Note that the speed is not the average of the speeds, 35 MPH.

There may be times when you need to adjust the correction factor after the end of the odo leg. You can mentally calculate small changes very easily. For each 0.01 mile you want to alter your mileage in 10 miles, change the factor by the value of the first digit of the factor. Increase the factor to increase your mileage; decrease the factor to decrease the mileage. Suppose your factor is 6021 and you want to increase your mileage 0.01 mile for each 10 miles you travel. Change the factor from 6021 to 6027 (6021+6). As another example, your factor is 4850 and you need a decrease of 0.02 mile per 10 miles. Change to 4840 (4850-4.8-4.8).


Imagine that you are at a checkpoint out-marker and are preparing for the next leg. You set the CTC and check the mileage then begin to pull ahead to clear the out-marker for other cars. As you do this, you notice that the CTC and odometer count almost immediately. When you were at the outmarker you had no way of knowing whether your odometer pulsed a few feet before you arrived there or whether it pulsed a foot prior to the marker. The same is true of the CTC. Of course it doesn't make any difference where they pulse before the marker, but it would be beneficial if the first mileage pulse occurs 52 feet into the next leg and the first time pulse occurs at the correct mileage for the speed in the computer. The 550 lets you insure that the pulses occur at the proper place. Push the safety before you leave the out-marker. The computer does the rest.

You will want to use this feature at the beginning of the odo leg, at the end of the odo leg, at the end of a transit zone, and whenever the rally's mileage is zeroed. Naturally, when you zero the odometer, you also pushed in the safety. You automatically make the next mileage pulse occur 52 feet later and the next time pulse goes in its proper place rise. Don't hold in the safety. The computer will act like it's in PARK.

As stated earlier in the manual, when pauses and gains are given in seconds, they must be converted to hundredths. Some values do not convert into even hundredths. Take for example, 10 seconds. In hundredths, this is 16.7. A pause of 10 seconds would be entered as 17. If you had a series of 10 second pauses, putting them all in as 17 would give you an error of 1 hundredth per three pauses. You can eliminate the problem by making the first two 17 and the third 16.

While all rally teams have a favorite technique for crossing a checkpoint line on time, you may find the following method best suited for the 550.

As you will soon notice, when you are at rally speed and are on time, it's a simple matter to hold the readout at 000. The readout will occasionally count down to 999 or up to 001. You can easily correct this by speeding up or slowing down. But what if the readout changes just before you cross the timing line? You won't have time to correct for it. Even when you can see the checkpoint in the distance, it's impossible to drive the exact rally speed. The readout may still change right at the line. As an alternative to driving at 000, try driving at 002. Now when you see the checkpoint, slow down to below rally speed, The readout can only count down in this situation. With a little practice, you can make the readout drop to 000 just before the timing line. When a checkpoint occurs without warning a tap on the brakes will lose the extra time.

PRICE LIST (Subject to change without notice)

Additional sending units and/or fittings can be purchased if you set-up another rally car.


Zeronics Engineering warrants all of its products to be free from defects in workmanship and material for a period of one year from original purchase. Our obligation under this warranty is limited solely to repairing any part when the product in returned to us within the warranty period providing: (1) the defective unit is returned to us transportation prepaid by purchaser, (2) no modification or change has been made to the unit's circuitry or wiring, (3) the unit has not been damaged by misuse, neglect, improper operation, accident, or alteration as determined by Zeronics Engineering. No other obligation is implied or expressed.


SPEED            SPEED             SPEED             SPEED

        1      60.00     16      3.750     31      1.935     46      1.304

        2      30.00     17      3.529     32      1.875     47      1.277

        3      20.00     18      3.333     33      1.818     48      1.250

        4      15.00     19      3.158     34      1.765     49      1.224

        5      12.00     20      3.000     35      1.714     50      1.200

        6      10.00     21      2-857     36      1.667     51      1.176

        7      8.571     22      2.727     37      1.622     52      1.154

        8      7.500     23      2.609     38      1.579     53      1.132

        9      6.667     24      2.500     39      1.538     54      1.111
       10      6.000     25      2.400     40      1.500     55      1.091

       11      5.455     26      2.308     41      1.463     56      1.071
       12      5.000     27      2.222     42      1.429     57      1.053

       13      4.615     28      2.143     43      1.395     58      1.034

       14      4.286     29      2.069     44      1.364     59      1.017

       15      4.000     30      2.000     45      1.333     60      1.000

                 OFFICIAL MILES                                  60              ELAPSED MINUTES
    NEW FACTOR = -------------- X OLD FACTOR    SPEED(mph) = ----------    MPM = ---------------
                 COMPUTER MILES                              MPM FACTOR           ELAPSED MILES


MIN.  SEC.      MIN.  SEC.      MIN.  SEC.      MIN.  SEC.

.01   0.6       .26   15.6      .51   30.6      .76   45.6

.02   1.2       .27   16.2      .52   31.2      .77   46.2

.03   1.8       .28   16.8      .53   31.8      .78   46.8

.04   2.4       .29   17.4      .54   32.4      .79   47.4

.05   3.0       .30   18.0      .55   33.0      .80   48.0

.06   3.6       .31   18.6      .56   33.6      .81   48.6

.07   4.2       .32   19.2      .57   34.2      .82   49.2

.08   4.8       .33   19.8      .58   34.8      .83   49.8

.083  5.0       .333  20.0      .583  35.0      .833  50.0

.09   5.4       .34   20.4      .59   35.4      .84   50.4

.10   6.0       .35   21.0      .60   36.0      .85   51.0

.11   6.6       .36   21.6      .61   36.6      .86   51.6

.12   7.2       .37   22.2      .62   37.2      .87   52.2

.13   7.8       .38   22.8      .63   37.8      .88   52.8

.14   8.4       .39   23.4      .64   38.4      .89   53.4

.15   9.0       .40   24.0      .65   39.0      .90   54.0

.16   9.6       .41   24.6      .66   39.6      .91   54.6

.167 10.0       .417  25.0      .667  40.0      .917  55.0

.17  10.2       .42   25.2      .67   40.2      .92   55.2

.18  10.8       .43   25.8      .68   40.8      .93   55.8

.19  11.4       .44   26.4      .69   41.4      .94   56.4

.20  12.0       .45   27.0      .70   42.0      .95   57.0

.21  12.6       .46   27.6      .71   42.6      .96   57.6

.22  13.2       .47   28.2      .72   43.2      .97   58.2

.23  13.8       .48   28.8      .73   43.8      .98   58.8

.24  14.4       .49   29.4      .74   44.4      .99   59.4

.25  15.0       .50   30.0      .75   45.0     l.00   60.0

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