Tire Size Calculator

Compare two tire sizes side by side: section width, aspect ratio, sidewall, overall diameter, circumference, revolutions per kilometre and speedometer error.

Compare two tire sizes side by side. The calculator returns overall diameter, sidewall height, circumference, revolutions per kilometre and the corresponding speedometer error — the four numbers that matter when fitting a non-OEM size.

Stay within ±1.5% diameter delta to keep the speedometer, odometer and ABS calibration accurate. Beyond ±3%, expect fault codes, mileage drift and clearance issues.

A

Current size (A)

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B

New size (B)

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How tire sizes work

Tire-size format Width (mm) / aspect (%) R rim (in). Example: 235/55R17 means 235 mm wide, sidewall = 55% of width, mounted on a 17-inch rim.
Speedometer error If the new tire is larger, the wheel rotates less per km and the speedometer under-reads. Stay within ±1.5% for daily drivers.
Overall diameter Calculated as (rim × 25.4) + 2 × sidewall, in millimetres. Real rolling radius is 96–98% of this static value due to load deflection.
Clearance A larger overall diameter eats wheel-well clearance. Always test full steering lock and full suspension compression before driving.
Rim width compatibility Each tire width has a recommended rim-width range (typically a 2.0″ window). Mounting outside this range distorts the tire profile, hurts handling and may void the warranty. ETRTO chart: 195 mm → 5.5–7″, 215 mm → 6.5–8″, 235 mm → 7.5–9″, 255 mm → 8.5–10″, 275 mm → 9–11″.
Load index & speed rating When changing tire size, the new tire's load index must equal or exceed the OEM rating, and the speed rating must equal or exceed it. Lower-rated tires are unsafe even if dimensions match. The full marking reads e.g. 235/55R17 99V — 99 = load index (775 kg), V = speed rating (240 km/h).

All calculations use the metric tire-size standard (width × aspect / 100 = sidewall mm). Real-world rolling diameter varies by ±1% with load, pressure and tread wear.