Speedometer Gear Calculator

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

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How to Calculate Speedometer Gear Teeth for Recalibration

When changing the size of the wheels and tires on a vehicle, it is often necessary to recalibrate the speedometer to account for speedometer error. If the vehicle has a mechanical speedometer, this means that replacing the driven gear will be necessary to recalibrate it.

The number of teeth needed for the new 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 number of teeth needed on the driven gear to properly recalibrate your speedometer, use the formulas below.

First, calculate the revolutions per mile for the new wheel and tire:

rev/mile = 20,168 / tire diameter (in inches)

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

Next, calculate the number of teeth for the new driven gear:

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

Thus, the number of teeth for the new driven gear is equal to the number of drive teeth multiplied by the axle gear ratio multiplied by the number of tire revolutions per mile, which is then divided by 1,001.

You might also be interested in our tire comparison calculator.

Frequently Asked Questions

What is a mechanical speedometer?

A mechanical speedometer, also known as an “eddy current” speedometer, is an instrument that displays a vehicle’s speed when in motion. It is operated by a gear-driven cable that is attached to the vehicle’s transmission.

What is the axle ratio?

The axle ratio describes the number of rotations that the vehicle’s driveshaft must make to turn the drive axle one complete revolution. The axle ratio will vary from vehicle to vehicle, based on its size, weight, powertrain, and intended purpose.

What does “the number of teeth” mean?

The number of teeth refers to the quantity of teeth on a particular gear.

What does revolutions per mile mean?

The revolutions per mile of a wheel and tire refer to the number of times that a given wheel and tire will revolve over a one-mile distance. Tire manufacturers may calculate this either in a laboratory setting or through other types of testing.