Find out how far off your speedometer reads after changing tire sizes. A speedometer counts wheel rotations and assumes a fixed tire diameter, so fitting taller tires makes the needle read slower than your true speed, while shorter tires make it read faster. Enter your original tire diameter, your new tire diameter, and the speed shown on the dial, and this calculator returns your actual speed along with the percentage error. This matters for avoiding tickets, keeping your odometer honest, and staying within the small error window carmakers design for. You can enter diameters directly from a tire spec sheet or from the tire size calculator. Even a two inch change in diameter can shift your real speed by several miles per hour. All math runs locally in your browser.
It compares the old and new tire diameters, since your true speed scales with the ratio of new diameter to old diameter.
A larger tire covers more ground per rotation, so at the same indicated speed the car is actually going faster than shown.
It gives your true speed at any indicated reading after the tire change, so you can gauge the error at highway speeds.
Find out how far off your speedometer reads after changing tire sizes. A speedometer counts wheel rotations and assumes a fixed tire diameter, so fitting taller tires makes the needle read slower than your true speed, while shorter tires make it read faster.
Yes. Speedometer Error Calculator is completely free, with no sign-up and no usage limits.
Yes. Speedometer Error Calculator runs in any modern web browser. There is nothing to download or install.
Yes. Speedometer Error Calculator runs entirely on your device in your browser, so nothing you enter is uploaded to a server.