# Speedometer Error Calculator

Enter the old and new tire size to calculate the speedometer error with the new tire.

## How to Calculate Speedometer Error

An important consideration when choosing new tires that vary in size from the stock tires on a vehicle is the accuracy of the speedometer. Speedometers work by measuring the number of revolutions of the axle to gauge the speed of the vehicle.

Larger wheels will rotate less to move the same distance while smaller wheels will rotate more. Because of this, as the overall diameter of a wheel changes so will the accuracy of the speedometer readings. This can usually be corrected, and that correction is an important step when increasing or decreasing the size of the tires on a vehicle.

It’s possible to estimate the speedometer error if you know the diameter of the new and the old tires. To calculate, find the ratio of the new tire diameter to the old one, then multiply by a speed on the speedometer to estimate the actual speed of the vehicle.

The formula to calculate the actual speed of the vehicle with a new tire size is:

actual speed = new diameter old diameter × speedometer

Thus, the actual speed of the vehicle will be the new tire diameter divided by the old diameter, times the speedometer reading. Try our tire comparison calculator to find the new and old tire diameters.

## How to Correct Speedometer Error After Changing Tire Size

Once you have identified there is error in the speedometer when installing a new tire size on a vehicle it’s time to correct it.

### How to Recalibrate an Electronic Speedometer

Every vehicle is different, but electronic speedometers can often be corrected pretty easily. To reset, you usually press the speedometer button, drive a prescribed distance, then press the button again to start the recalibration.

The first step is often to look up the process for resetting the speedometer in the documentation from the manufacturer. This is also where the driving distance will be documented.

When you have the driving distance, start the process by pressing and holding the speedometer calibration button, then start the vehicle, and then release the button.

Then, press the button again.

Drive the prescribed distance.

Press the button a final time to recalibrate the speedometer to the new tire size.

### How to Recalibrate a Mechanical Speedometer

The process for recalibrating a mechanical speedometer is a little more involved, and requires identifying the number of teeth needed for the new driven gear and replacing the gear with the new one.

Start by looking at the vehicle documentation to find the axle ratio.

Then, determine the overall diameter of the new tire. You can do this using our tire size calculator or simply by measuring it. Then, divide the diameter by 20,168 to find the revolutions per mile. Our tire size calculator will also supply this value.

Then, open the hood and locate the speedometer cable attached to the transmission’s tail shaft. Unscrew the cable from the transmission, then remove the gear housing cover to expose the gears.

Identify the drive gear and count the number of teeth on it.

Now it’s time to size the new gear. To do so, multiply the revolutions per mile of the new tire by the number of drive gear teeth. Then, multiply that by the axle ratio. Finally, divide the result by 1,001 to find the number of gear teeth needed on the new driven gear.

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

You can also use our speedometer gear size calculator to find the right number of teeth.

Locate a new driven gear with the correct number of teeth. You will need to get the specific gear that will fit your vehicle.

Replace the driven gear with the new one. Reinstall the housing cover and reattach the speedometer cable to the transmission.