Speedometer Gear Calculator
Calculate the correct driven gear tooth count for mechanical speedometers after changing tire size or differential ratio.
Driven Gear Teeth Needed
23 teeth
Tire Revs/Mile
776
Drive Gear
8 teeth
๐ก Formula: Driven Teeth = (Revs/Mile ร Axle Ratio ร Drive Teeth) รท 1,000
๐ก How to Use the Speedometer Gear Calculator
Speedometer Gear Sizing
Vehicles with cable-driven speedometers (pre-2000s vehicles, many trucks and muscle cars) use a pair of gears in the transmission tailhousing to drive the speedometer cable. When you change tire size or differential ratio, you need a new driven gear to maintain accuracy.
Driven Teeth = (Tire Revs/Mile ร Axle Ratio ร Drive Teeth) รท 1,000
The drive gear is attached to the output shaft and stays fixed. The driven gear (the replaceable plastic gear) meshes with it and spins the speedometer cable.
Common Drive Gear Tooth Counts
| Vehicle | Drive Teeth |
|---|---|
| GM TH350/TH400 | 7 or 8 |
| GM 4L60E/4L80E | 15 |
| Ford C4/C6/AOD | 7 or 8 |
| Ford 4R70W | 7 |
| Chrysler A727/A518 | 8 |