Tire Size Speedometer Accuracy Calculator

Calculate the change in speedometer accuracy when tire sizes are changed with this online calculator.

For a passenger vehicle, the tire size can be found branded on the sidewall of a tire. For an example, a passenger tire may have a size that looks similar to P225/45R16. The P is the designation for passenger tire, and the R represents that it is a Radial.

Original Tire Size = P / R

New Tire Size = P / R

Fill in the numbers posted on the sidewall of a tire to estimate the change in speedometer accuracy.

Why does changing the tire size affect the speedometer?

Most vehicles have a speedometer that is calibrated to the tire size that came from the factory. If you change the size of the tire, you alter the total circumference and the amount of travel per rotation.

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