Speedometer Error After Tire Change Calculator

About this calculator

A speedometer is calibrated to the exact rolling circumference of the original-equipment tire, counting wheel revolutions and converting them to speed on that assumption — so fitting a tire with a different overall diameter (through a wheel size change, a different aspect ratio, or plus-sizing) makes every revolution cover a different actual distance than the speedometer expects. True speed equals the indicated speed multiplied by the ratio of the new tire diameter to the old one: a larger tire means you are actually going faster than the speedometer shows, and a smaller tire means you are going slower.

Car enthusiasts changing wheel or tire sizes — especially when "plus-sizing" to larger wheels with a lower-profile tire — use this calculation to know how far off their speedometer and odometer will read after the swap, since a large enough diameter change can create a meaningful, sometimes legally relevant, gap between indicated and actual speed.

This calculator takes your old and new tire sizes plus an indicated speed and returns your true speed and the exact speedometer error, so you can know what you are actually driving after a tire size change.