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. 205/50-R15).
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.
Editor's 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.