Tire Size & Speedometer Error Calculator
Compare two tire sizes (e.g. 225/45R17 vs 235/40R18): diameter, width and sidewall differences, revolutions per mile and the speedometer error after changing tires.
How to use
- 1Enter your current tire size and the new size (as printed on the sidewall).
- 2Keep the overall diameter within about 3% to avoid speedometer and ABS issues.
How it's calculated
Sidewall = width × aspect ratio ÷ 100. Diameter = rim × 25.4 + 2 × sidewall (mm). Actual speed = indicated speed × new diameter ÷ old diameter.
Frequently asked questions
How much diameter difference is OK?
Most guides recommend staying within ±3% of the original diameter to keep speedometer, ABS and stability control accurate.
What does 225/45R17 mean?
225 mm wide, sidewall 45% of the width, radial construction, for a 17-inch rim.
Will bigger tires make my speedometer read low?
Yes — larger tires cover more ground per revolution, so you're going faster than the speedometer shows.