Tire Size Difference Calculator
Compare two tire sizes for overall diameter, circumference, and speedometer error if you change width, aspect, or rim.
How to use this tire size difference calculator
Tire size difference matters when you plus-size wheels or swap a spare. Diameter change moves speedometer, odometer, ABS calibration, and fender clearance. Staying inside about 3% overall diameter is the usual DIY rule; more than that wants a shop and maybe a reprogram.
Width and aspect can trade off: a lower profile on a larger rim often keeps diameter close. This calculator does not check load index, speed rating, or snow vs all-season. Read the door sticker before you buy.
Formula
Worked example
225/45R17 vs 245/40R18: plus-sizing often stays within ~3% diameter. If the new tire is taller, the speedometer reads low (you are going faster than the needle).
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Frequently asked questions
How do I compare two tire sizes?
Convert each to overall diameter from width, aspect, and rim. Then compare circumference and the percent change.
Will my speedometer be wrong?
If the new tire is taller, the speedo reads slower than true speed. If shorter, it reads fast.
What does 225/45R17 mean?
225 mm tread width, 45% aspect (sidewall is 45% of 225), R = radial, 17-inch rim.
How much diameter change is OK?
About 3% is a common limit for speedometer and gearing. Check the vehicle maker for AWD and TPMS quirks.
Does width change diameter?
Only through the sidewall (width × aspect). A wider tire with a lower aspect can stay the same height.
Results are estimates for informational purposes and are not financial, legal, or professional advice. Last reviewed 2026-08-20.