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Speedometer Gear Calculator

Calculate the correct speedometer driven gear.

Drive Gear Teeth

Axle Ratio

Tire Diameter (Inches)

inches

Enter the total height of your tire in inches.

1Why calculate this?

When you change your vehicle's tire size or rear axle gear ratio, your speedometer will become inaccurate. Changing the driven gear in your transmission is a common way to recalibrate it.

2How to choose?

The calculator provides the ideal tooth count. Since gears only come in whole numbers, choose the available gear with the tooth count closest to the calculated value.

Features

Gear Calculation

Calculate driven gear teeth based on axle ratio and tire size.

Tire Revs

Estimate tire revolutions per mile.

Accurate Results

Get precise tooth counts for speedometer calibration.

About Speedometer Gear Calculator

The Speedometer Gear Calculator is designed for automotive enthusiasts and mechanics who have changed their vehicle's tire size or rear axle gear ratio. These changes often lead to an inaccurate speedometer. This tool helps you determine the correct driven gear tooth count to recalibrate your speedometer, ensuring accurate speed readings.

How to Use Speedometer Gear Calculator

1
Enter Drive Gear Teeth
Input the number of teeth on your transmission's drive gear.
2
Enter Axle Ratio
Input your vehicle's rear axle gear ratio (e.g., 3.55, 3.73, 4.10).
3
Enter Tire Diameter
Input the total diameter of your rear tires in inches.
4
View Result
See the calculated driven gear tooth count and choose the closest available gear.

Frequently Asked Questions

The speedometer is calibrated for a specific tire diameter. Larger tires travel further per revolution, making the speedometer read slower than your actual speed.

It's the small plastic gear at the end of the speedometer cable that plugs into the transmission. Changing it adjusts how fast the cable spins.

You can find it on a metal tag on your differential housing, on the door jamb sticker, or by counting driveshaft vs. wheel rotations.

This is for traditional gear-driven speedometers. Modern digital ones usually require an electronic calibrator or ECU tuning to fix accuracy.

Since you can't have half a tooth, you must choose either 19 or 20. 19 will read slightly fast, while 20 will read slightly slow.

Usually, changing the Driven Gear is enough. You only change the internal Drive Gear if the required Driven Gear doesn't exist (too many or too few teeth).