# Speedometer Error Calculator

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

Tire One
/ R
Tire Two
/ R

## Actual Speed:

mph

kph
Learn how we calculated this below

## How to Calculate Speedometer Error

A speedometer is an instrument found on a vehicle’s dashboard that displays its speed when in motion. While the first car equipped with a speedometer was the 1901 Oldsmobile, it took until the 1920s for speedometers to become standard equipment on every vehicle produced.

Most speedometers are accurate when using the same size wheels and tires that the vehicle left the factory with. If you modify the wheels and/or tires so that their diameters are different from the original equipment versions, there will likely be some error in your speedometer readings.

This is because most speedometers work by measuring the number of revolutions of a gear located in the transmission. This gear provides a reading that translates to the speed of the vehicle, based on the number of revolutions of the original wheels and tires.

Compared to the originals, larger-diameter wheels and tires will rotate fewer times to move the same distance, while smaller-diameter wheels and tires will rotate more times. This is why changing the size of your wheels and tires affects the accuracy of your speedometer readings.

The good news is that this speedometer error can usually be corrected. It should be done to assure you of accurate speedometer readings after you increase or decrease the diameter of your wheels and tires.

### Actual Speed Formula

It is possible to estimate your speedometer error if you know the diameters of both the new and the old wheels and tires. To calculate the error, find the ratio of the new wheel and tire diameter to the old one, then multiply by the 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 an error in your speedometer after installing different-sized wheels and/or tires on your vehicle, it’s time to correct it by recalibrating your speedometer.

### How to Recalibrate an Electronic Speedometer

Every vehicle is different, but electronic speedometers can usually be easily recalibrated.

The first step is to check your owner’s manual (or manufacturer documentation if it is an aftermarket speedometer) for the process involved and the distance that needs to be driven to successfully recalibrate your electronic speedometer.

The recalibration process usually works like this (the exact details may vary slightly):

With the power off, push and hold the calibration button on the speedometer.

Continue holding the button, start the vehicle, then release the button.

Drive to the place where you will drive the prescribed distance. Come to a complete stop, but do not turn off the ignition.

Push the button again and drive the prescribed distance. Come to a complete stop and push the button once more. Your recalibration is now complete.

### How to Recalibrate a Mechanical Speedometer

A mechanical speedometer is also known as an “eddy current” speedometer. This type of mechanical speedometer was patented in 1902 by Otto Schulze. It remained in widespread use until the 1980s, when electronic speedometers began to appear in more and more vehicles.

A mechanical speedometer is operated by a gear-driven cable that is attached to the vehicle’s transmission.

The process of recalibrating a mechanical speedometer involves replacing the existing gear (called the “driven gear”) with a new driven gear that has the correct number of teeth to compensate for the difference in the new wheel and tire diameter. This will correct the speedometer error created by the change in wheel and tire size.

Start by finding the vehicle’s axle ratio in the owner’s manual or other vehicle documentation.

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

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

Find the driven gear and count the number of teeth on it.

To determine the size of the new driven gear, multiply the revolutions per mile of the new wheel and tire by the number of driven gear teeth. Then, multiply that by the axle ratio.

Finally, divide the result by 1,001 to find the number of gear teeth that will be 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 for the new driven gear.

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

Replace the existing driven gear with the new one. Reinstall the gear housing cover and reattach the speedometer cable to the transmission. The recalibration process is finished.