Tire Size Calculator

Compare two tire sizes to see the difference in diameter, sidewall height, circumference, and speedometer accuracy.

About the Tire Size Calculator

Compare two tire sizes to see how a size change affects overall diameter, sidewall height, and speedometer accuracy.

How the Tire Size Calculator works

A tire size like 225/45R17 encodes three numbers: section width in mm (225), aspect ratio as a percentage of width (45% sidewall height), and rim diameter in inches (17). Changing any of these changes the tire's overall rolling diameter — and since your speedometer is calibrated for the factory tire's diameter, a meaningfully larger or smaller tire will make your speedometer and odometer read slightly off from your true speed and distance.

How to use the Tire Size Calculator

Enter your original tire size, e.g. 225/45R17.
Enter the new tire size you're considering, e.g. 235/40R18.
Press Calculate to see the diameter, sidewall height, circumference, revolutions per mile, and speedometer difference for both.

Tips for the Tire Size Calculator

Tire shops generally recommend staying within about 3% of the original overall diameter to avoid speedometer error and clearance issues.

Formula used by the Tire Size Calculator

Sidewall height (mm) = Width × (Aspect Ratio ÷ 100)
Overall diameter (in) = Rim diameter + 2 × (Sidewall height ÷ 25.4)
Revolutions per mile = 63360 ÷ (Diameter × π)

Example: Tire Size Calculator

Comparing 225/45R17 to 235/40R18

Original size: 225/45R17
New size: 235/40R18

Result: Diameter 24.97 in → 25.40 in (about 1.72% larger)

Original diameter = 17 + 2×(225×0.45÷25.4) ≈ 24.97 in. New diameter = 18 + 2×(235×0.40÷25.4) ≈ 25.40 in — within the ~3% range tire shops generally consider safe.