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. San Diego Speedo-Tach is not responsible for any miscalculations of this calculator or any errors as a result of curvature effects of the tire.

Please input your data here:
Enter your stock tire size :
 / - R 		Enter the new tire size :
 / - R
Please enter a speedometer reading: MPH or KM/H

Here is your calculated data:
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 your speedometer indicates MPH / KM/H, you would actually be travelling MPH or KM/H.