Tire Size Calculator — Diameter & Speedometer Error Comparison

Changing tire sizes — going up a wheel diameter, fitting wider rubber, or swapping to an off-road flotation size — changes more than looks. It changes the tire's overall diameter, which changes how far your car travels per wheel rotation, which in turn changes what your speedometer and odometer report. This calculator answers exactly how much, from the tire codes alone.

Enter your current tire size and the one you are comparing it to. Both the metric convention printed on nearly every passenger car tire (section width in millimeters / aspect ratio as a percentage / rim diameter in inches, like 225/45R17) and the flotation convention common on trucks and off-road tires (overall diameter x section width R rim diameter, like 33x12.5R15) are recognized automatically — mix and match the two formats freely, the tool converts between them internally.

For each tire you get the sidewall height, overall diameter and circumference in both millimeters and inches, plus revolutions per mile and per kilometer and the wheel's RPM at a speed you choose. The comparison block then reduces the two sizes to what actually matters when you are deciding on a fitment: the diameter difference in inches and percent, and the resulting speedometer error — whether the new tire will make your speedometer read fast or slow, and by roughly how much.

The math follows the same formula tire shops and off-road forums use: sidewall height = section width × aspect ratio, overall diameter = rim diameter + 2 × sidewall height. Real-world diameters can vary a percent or two between manufacturers even for the same nominal size, so treat plus-size and off-road fitment decisions as a close estimate rather than a guaranteed exact match. For converting the resulting distances to other units, see the length converter; everything here runs locally in your browser.