Tire Size Calculator
Decode any tire size and compare two sizes: diameter, sidewall, revolutions per mile and speedometer error, in mm and inches.
How it works
A size like 225/45R17 means 225 mm wide, sidewall 45% of the width, on a 17-inch wheel.
We compute sidewall height, overall diameter, circumference and revolutions per mile, then compare the new size to your current one.
A to-scale drawing shows both tires side by side. If diameters differ by more than 3%, we flag it, your speedometer, ABS and clearance could be affected.
Formula
sidewall
Where:
- width
- = Section width in mm
- aspect
- = Sidewall height as % of width
- rim
- = Wheel diameter in inches
Worked example
225/45R17: sidewall = 101.25 mm, diameter = 431.8 + 202.5 = 634.3 mm (24.97″), about 807.6 revolutions per mile.
235/45R17 measures 643.3 mm, 1.42% larger. At an indicated 60 mph you're really going about 60.9 mph.
Practical Tips for Tire Upsizing
When switching tire sizes, the most critical factor is maintaining a diameter within 3% of the factory specification. This ensures ABS, electronic stability control, and transmission shift points function correctly. Going larger may cause 'gear hunting' in automatic transmissions. Always check the load index and speed rating on your new tires to ensure they meet the requirements on your door jamb sticker.
Consider physical clearance inside the wheel well. A tire might fit while parked but rub against the fender liner during sharp turns or suspension compression. If increasing width significantly, you may need to adjust the wheel offset to prevent the inside sidewall from contacting the strut. Regular pressure checks are vital, as larger tires often require different PSI settings for even wear.
Common Mismatches and Drivetrain Risks
Mixing tire sizes on a 4WD or AWD vehicle is a dangerous mistake. Even a small diameter difference between axles can cause the center differential to overheat and fail. If replacing tires on an AWD car, replace all four at once or ensure the new ones are shaved to match the tread depth of the remaining tires to keep rolling circumferences identical.
Forgetting the spare tire is a frequent oversight. If you upgrade to a larger size but leave the factory spare, using it during a flat could damage your drivetrain or trigger dashboard warnings. Remember that a taller tire changes your final drive ratio, making your speedometer read lower than your actual speed. Use a GPS app to verify your true velocity after a significant size change.
Frequently asked questions
Most experts recommend staying within ±3% of the original diameter to keep speedometer, ABS and stability systems accurate.
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For informational purposes only. This calculator does not provide professional engineering advice. Consult a qualified professional before making decisions based on these results.