Tire size calculator
Compare two tire sizes (such as 205/55R16) to see the difference in diameter and sidewall height, and the impact on speedometer accuracy before you change your tire setup. Fully local calculation, in your browser.
How it works
A tire size such as "205/55R16" reads as follows: 205 mm wide, an aspect ratio (height to width) of 55%, mounted on a 16-inch rim. From these three numbers, the tool calculates the sidewall height, the overall tire diameter and its circumference, then compares two sizes with each other to estimate the diameter difference and its impact on the speed actually shown on the speedometer.
Why does changing tire size affect the speedometer?
The speedometer is calibrated to the original circumference of the tire and the number of wheel revolutions per minute. If the new tire has a larger diameter, the car covers more distance per wheel revolution than the speedometer assumes, so it will show a speed slightly lower than the actual speed (and the reverse for a smaller tire).
What diameter difference is considered acceptable?
The generally accepted automotive standard tolerates a total diameter difference of less than about 3% compared with the original setup, so as not to significantly skew the speedometer, ABS or stability control. Beyond that, it is recommended to consult a professional before changing tire size.
This comparison is a simple geometric calculation performed in your browser, with no data transmitted.