Tyre Size Calculator
Decode markings like 225/45R17 into real dimensions, and compare two tyre sizes side by side.
Read the markings on your tyre sidewall — e.g. 225/45R17
Tyre A
Tyre B — compare (optional)
Tyre A
Tyre B
About the Tyre Size Calculator
Tyre sidewall markings encode three key measurements: width, aspect ratio, and rim diameter. This calculator decodes them into real-world dimensions — outer diameter, sidewall height, and circumference — and compares two sizes to check compatibility.
Understanding the comparison
When fitting a different tyre size, the outer diameter should stay within 1–3% of the original. A larger diameter means the speedo reads low (actual speed is higher than displayed). A smaller diameter means the speedo reads high.
Frequently Asked Questions
How do I read tyre markings?
225/45R17: 225 = width in mm, 45 = aspect ratio (sidewall as % of width), R = radial, 17 = rim diameter in inches.
How much size difference is acceptable?
Stay within 3% of original diameter. Under 1% is ideal. Over 3% risks speedo error, clearance issues, and ABS calibration problems.
Why does tyre size affect the speedo?
Speed is calculated from wheel revolutions. A larger tyre covers more distance per revolution, so the speedo reads lower than actual speed.
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