Freqently Asked Questions (FAQs)
Speedometer Gear Calculator

by George Davenport -

Please Select:
  • Drive Gear Teeth
  • Rear End Ratio
  • Tire Height
  • Drive Gear Teeth
    NOTE: SC manual transmissions (M5R2) '89-'95 and automatic tranmissions (AOD) '89-'93 use an 8 tooth drive gear. SC automatic transmissions (4R70W) '94-'95 use a 7 tooth drive gear.

    Rear End Ratio
    Tire Height (inches)
    You can calculate tire height on the Tire Height Calculator

    Speedometer Gear Teeth

    This calculation is based on the following formula:

    Speedometer Gear Teeth = (Drive Teeth * Rear End Ratio * Tire Revolutions per Mile) / 1,000

    The constant "1,000" is used since it is the RPM of the Thunderbird SC speedometer at 60 MPH. That is the current standard for most modern automobile speedometers.

    SC manual transmissions (M5R2) '89-'95 and automatic tranmissions (AOD) '89-'93 use an 8 tooth drive gear. SC automatic transmissions (4R70W) '94-'95 use a 7 tooth drive gear.

    The formula may also be simplified into this format:

    Speedometer Gear Teeth = (Drive Teeth * Rear End Ratio * 20.2) / Tire Height


       '89 SC with AOD transmission, 245/45/17 tires and 3.73 differential gears

    Tire Height from Tire Height Calculator: 25.68 - rounded to 25.7

    (8 * 3.73 * 20.2)/25.7 = 23.4 tooth speedo gear


    When measuring tire height, it is best to get the actual height from the manufacturer's specifications for your tire.

    This calculator will give the number of speedometer gear teeth to the nearest "tenth" of a tooth to let you determine which closest whole tooth number would be best for your application. You can also determine the effects of tire wear by varying the tire height in 0.1" increments.

    The following chart lists currently available speedometer gears and their part numbers.

    (Chart last updated on 1/11/1999).


    Application Teeth Color Part #
    SC Auto 16 Blue D0AZ-17271-A
    SC Auto 17 Green C7SZ-17271-A
    SC Auto 18 Grey C7SZ-17271-B
    SC Auto 19 Tan C7VY-17271-A
    SC Auto 20 Orange C8SZ-17271-B
    SC Auto 21 Purple D0OZ-17271-B

    SC Manual 19 Pink C0DZ-17271-B
    SC Manual 20 Black C1DZ-17271-A
    SC Manual 21 Red C4OZ-17271-A

    Cyberdyne Speedometer Calibration Box
    Cyberdyne Part # 9902
    Purchased from Jegs, priced about $70

    Calibrate Your Speedometer for Higher Gear Ratios
    (or When a 23 Tooth Gear Just Won't Do!)

    Installed the unit and it works great. The antilock brakes still work and I was even able to change the speed to a lower than accurate speed. Finally, had my girlfriend drive in front of me at a set speed and both used cell phones to get it right.

    I had a hard time finding the wire to split the speedometer signal wire inside the car. The best place I found is removing the console and in the right front there is a connector. On a 92 SC, it is the greeen/white wire. On the calibrator, the white wire comes from the speedometer gear sensor then the blue (not blue with black stripe) wire goes on to the back of the computer and the speedometer. Hook up red wire to (+) and black wire to (-).

    Have a buddy turn the coarse potentiometer until you get it close or right on. I didn't even need to use the fine adjustment. This is nice because you can change tire size, gears, etc. it was about $70 from JEGs and the company was Cyberdyne. The model was 9002. Just tell them it's the one that is for the stock gauge not the cyberdyne gauge.