This calculator calculates speedometer error and tire sizes based on theoretical tire dimensions as given by the standard US tire system (i.e. 205/45-ZR16) and assumes a new tire where the numbers on the tire accurately represent the dimensions. The calculator DOES NOT take into effect any curvature effects of a tire when placed on a rim it is not designed for. For example, a 205 width tire mounted on a 15" diameter, 5" wide wheel will bulge since the tire is designed to be flush with a 7-7.5" wide wheel. Naturally the tire will balloon outward from the sides, which is not taken into consideration (calculating curvature effects requires some heavy duty calculus.) This calculator gives the user a general idea of wheel effects. Palo Alto Speedometer is not responsible for any miscalculations of this calculator or any errors as a result of curvature effects of the tire.