Tire Size Calculator
Compare two tire sizes to see the change in overall diameter and speedometer accuracy.
Assumptions used in this calculation
- ISO tire-code formula: Overall diameter uses the standard ISO metric tire-code formula (rim diameter in inches plus twice the sidewall height derived from width × aspect ratio); it doesn't account for manufacturer variation in actual tire dimensions.
About this calculator
Fitting a different tire size, common when upgrading wheels or choosing an alternative brand, changes the tire's overall rolling diameter in ways that aren't obvious from the size code alone, and a large enough change throws off both your speedometer and odometer. This calculator takes the width, aspect ratio and rim diameter for both your original and new tire sizes, computes overall diameter for each using the standard ISO formula (rim diameter plus twice the sidewall height derived from width times aspect ratio), and reports the percentage difference. It's useful before buying replacement tires or a different wheel/tire package, so you can check the size stays close enough to stock that your speedometer reading and any ABS/traction-control systems that rely on wheel speed remain accurate. Working out sidewall height and diameter differences by hand involves several unit conversions between millimetres and inches, which this calculator handles automatically so you just compare the two size codes directly.
Worked example
205/55R16 vs 225/50R17
Result: +3.94% diameter
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