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Tire Size Calculator
Enter a tire size to get overall diameter, sidewall, circumference and revs per mile.
Overall diameter
—
Tire + wheel.
| Sidewall height | — |
|---|---|
| Circumference | — |
| Revolutions / mile | — |
| Section width | — |
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Enter your figures above to see the step-by-step working.
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How the Tire Size Calculator Works
A tire size like 225/45R17 packs three numbers: the tread width in millimetres (225), the aspect ratio — sidewall height as a percentage of width (45%) — and the wheel diameter in inches (17). This calculator turns those into the overall tire diameter, sidewall height, circumference and revolutions per mile.
That overall diameter matters when changing tire sizes: a taller or shorter tire changes your speedometer reading, gearing and ride height. Compare two sizes by running each through the calculator and checking the diameter and revs-per-mile difference.
diameter = wheel + 2 × (width × aspect% ÷ 25.4)
Frequently Asked Questions
What do the numbers in a tire size mean?
Width in mm / aspect ratio % / R (radial) wheel diameter in inches. So 225/45R17 is 225 mm wide, sidewall 45% of that, on a 17-inch wheel.
How does tire size affect my speedometer?
Your speedometer is calibrated to the original diameter. A larger overall diameter makes you actually go faster than indicated; a smaller one, slower.
What is revolutions per mile?
How many times the tire turns to cover a mile (63,360 inches ÷ circumference). It affects gearing and speedometer calibration.
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