by George Davenport - email@example.com
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
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).
Calibrate Your Speedometer for Higher Gear Ratios
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.