How to Use Speedometer Gear Calculator
The Speedometer Gear Calculator supports two modes.
Recommend Mode
- Enter Tire Diameter — Measured in inches.
- Enter Axle Ratio — Such as 3.73, 4.10, or 3.55.
- Enter Drive-Gear Teeth — The fixed transmission gear tooth count.
- Read the Result — The Speedometer Gear Calculator returns the ideal driven-gear teeth count, rounded to the nearest whole number for ordering.
Error Mode
If you already know how off your speedometer reads, enter the current driven-gear teeth and the percent error. The calculator suggests how many teeth to add or remove.
Formula & Theory — Speedometer Gear Calculator
driven_teeth = (tire_diameter × axle_ratio × drive_teeth) ÷ KFor most US passenger transmissions K is about 20168, derived from 1001 revolutions per mile on a 26-inch tire reference. The Speedometer Gear Calculator uses the standard automotive constant.
| Symbol | Meaning |
|---|---|
| tire_diameter | Loaded tire diameter (in) |
| axle_ratio | Differential ratio |
| drive_teeth | Transmission output drive-gear teeth |
| driven_teeth | Replaceable driven-gear teeth |
Error Adjustment
new_teeth = current_teeth × (1 + error% / 100)A speedometer that reads 5% high needs about 5% more driven-gear teeth to read correctly.
Use Cases for Speedometer Gear Calculator
- Bigger Tire Upgrades — Recalibrate the speedometer after off-road or lift kits.
- Axle Swaps — Match the speedometer to a new differential ratio.
- Restomod Builds — Pair classic transmissions with modern tires.
- Inspection Compliance — Many regions require speedometer accuracy within a small tolerance.
- Fuel Economy Reporting — Accurate speedometer & odometer readings give better MPG numbers.
The Speedometer Gear Calculator keeps your speed and odometer honest after any drivetrain change.