# Speedometer Gear Calculator

Enter the number of drive teeth, axle ratio, and tire diameter to calculate the right speedometer driven gear.

## How to Calculate Speedometer Gear Teeth

When changing the size of wheels and tires on a vehicle it’s often necessary to recalibrate the speedometer to account for speedometer error. For a manual speedometer that means replacing the driven gear.

The number of teeth needed for the driven gear will vary depending on the size of the tires, the vehicle’s axle ratio, and the number of drive teeth in the speedometer. To calculate the size of the gear needed, use the formulas below.

First, calculate the revolutions per mile for the wheel:

rev/mile = 20,168 / tire diameter

Try out tire size calculator to find the diameter of the tire.

Then, calculate the number of teeth for the driven gear:

driven teeth = drive teeth × axle ratio × rev/mile / 1,001

Thus, the number of driven teeth is equal to the number of drive teeth times the axle ratio times the tire revolutions per mile divided by 1,001.

You might also be interested in our tire comparison calculator.