It determines speedometer error and tire size information based on published and generally accepted tire dimensions as given by the standard U.S. 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. The 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.
This tool gives the user a general but reasonable representation of alternative wheel effects. Enter the standard and new tire designation, aspect and wheel size. Click on Calculate for the results. You may click on Clear Values to do another calculation. Consult your tire specialist if you have further questions about such issues.
Please be advised: The accuracy of this calculator and its applicability to your circumstances is not guaranteed. It's meant to be used as an informational guide only. Always obtain professional advice from qualified automotive professionals.