Tire Size Calculator
Compare two tire sizes side by side. See the real diameter difference, how far your speedometer will be off, and a to-scale drawing of both tires.
Enter Two Tire Sizes
Tire Comparison Results
| Measurement | Current | New | Difference |
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Speedometer Error
| Speedometer shows | True speed | Error |
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How the numbers are calculated
A size like 225/45R17 reads as: 225 mm section width, a sidewall that is 45% of that width, on a 17 inch rim. Overall diameter is the rim plus two sidewalls: rim + 2 x (width x aspect / 100) / 25.4. Circumference is diameter x pi, and revs per mile is 63,360 inches divided by the circumference. That diameter feeds straight into gearing - the gear ratio calculator accepts sizes in this same format and shows what the change does to engine rpm.
Speedometer and odometer error follow directly from geometry. The car counts wheel revolutions and assumes the original circumference, so a taller tire means every revolution covers more road than the car thinks: true speed is higher than indicated and the odometer records fewer miles than you actually drive. The error percentage equals the diameter change percentage.