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Speedometer Calibration / Tire Size Change Calculator

This calculator is designed to give information related to tire size changes and the general impact they have on the calibration of a speedometer. It determines speedometer error and tire size information based on published and generally accepted tire dimensions as given by the standard US tire system (i.e. 205/45-ZR16). All calculations are based on the assumption of a new tire (as opposed to used) where the numbers on the tire accurately represent the dimensions.

U.S. Tire Information
U.S. Tire Required Data Entry
Enter Stock Tire Size / - R	Enter New Tire Size / - R
Enter A Reference Speedometer Value MPH or KMH

Calculated Results After Tire Changes
Stock tire width is inches.	New tire width is inches.
Stock tire thickness is inches.	New tire thickness is inches.
Stock tire diameter is inches.	New tire diameter is inches.
When the vehicle speedometer indicates MPH or KMH.
The vehicle speed is actually MPH or KMH.

NOTE: This calculator makes no effort to consider the result of any curvature effects of a tire when mounted on a rim it was not designed for. As an example, a 205 width tire mounted on a 15 inch diameter, 5 inch wide wheel will bulge significantly since the tire is designed to be flush with a 7 to 7.5 inch wide wheel. Under that stress, the tire will balloon outward from the sides, which is not taken into consideration within the limitations of this calculator.

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