In most vehicles, the speedometer calibration is based on a certain tire and transmission ratio. These factors were used by the speedometer manufacturer to establish the proper combination of gears and worms inside the speedometer head to obtain accurate speed readings. Some car manufacturers and individuals building customized cars are using different tires and/or transmission ratios rather than OEM equipped. If this is the case, the ratio has to be established. In general, if the tires or transmission, including differential of your car has been or will be altered by you, or if any other changes have been made that are not listed as original or factory approved equipment, the ratio has to be adjusted. The following tests will tell you if the error you are experiencing is in the speedometer gauge itself or is caused by cable delivery from the transmission.

THE EASY WAY TO TEST RATIO

1. Stop at a mile marker found on most US highways. Record the odometer reading, including the 10ths.
2. Drive 10 miles and stop at the mile marker. Record the odometer reading again, including the 10ths.
3. Subtract the first reading from the last reading. This will tell us if the cable delivery (ratio) is correct or incorrect. If correct, your subtraction answer will be 10.0.

Conclusion: If odometer reads correctly per 10 miles driven, the error you are experiencing is in the speedometer head. If reading is incorrect, the error is from your transmission and the ratio must be corrected. Although rare, you could have both cable delivery and speedo head problems. In any event your cable delivery must be correct in order for the speedometer to be accurate. Contact APT.

FOR AMERICAN MADE CARS Generally, American made vehicles with mechanical speedometers need 1000 cable revolutions per mile to make the odometer record correctly and the MPH to be accurate. Here's a simple formula for you to fix many American made car ratio problems. Conduct the 10 mile test, remove driven gear (the gear that spins your cable), count the teeth, and then multiply the teeth times the subtraction answer from the test.

Example: 8.2 reading per ten miles actual multiplied by 38 teeth on driven gear (8.2 X 38) = 311.6, which means you need a 31 tooth gear to correct the error. In other words, 8.2 miles per 10 miles actual is 820 cable revolutions and you need 1000 revolutions to be correct. If you find that you cannot buy a driven gear with the right number of teeth, contact APT who can build a ratio adapter to fix the problem.


THE HARD WAY TO TEST RATIO

1. Check if the tire pressure is the same as advised by the factory.
    
2. Measure (by making chalk marks) a distance of 52'9-1/2" - 16.09m.
    
3. Unscrew speedometer cable at the speedometer head.
    
4. Mark the inner core with a paper flag, paper clip, or preferably a hairpin.
    
5. Push or drive vehicle over the entire distance, counting the full and partial revolutions of the cable.
    
6. Always roll car towards the first marking until you see the inner core start turning to eliminate the gear clearance. Before you start counting, mark the part of the vehicle that is exactly over the beginning of the line. Stop counting when the marked portion of the vehicle is exactly over the end of the line.
    
7. Repeat this procedure three times and jot down the results:
    
   1. Test___________________ full__________________ partial turns.
       
   2. Test___________________ full__________________ partial turns.
       
   3. Test___________________ full__________________ partial turns.