How it works
Metric tire sizes mix units: width in millimeters, sidewall as a percentage of width, rim in inches.
Sidewall (mm) = width × aspect ratio ÷ 100
Diameter (mm) = rim (in) × 25.4 + 2 × sidewall
Revolutions per mile = 63,360 ÷ (π × diameter in inches)
Actual speed = indicated speed × new diameter ÷ original diameter
Worked example
Swapping 275/65R18 tires for 285/70R17.
- 275/65R18: sidewall 178.8 mm, diameter 814.7 mm (32.1 in)
- 285/70R17: sidewall 199.5 mm, diameter 830.8 mm (32.7 in)
- Difference: +2.0%. At an indicated 60 mph you are doing about 61.2 mph.
Tire size chart
| Size | Diameter | Sidewall | Revs per mile |
|---|---|---|---|
| 235/75R15 | 28.9 in (734 mm) | 6.9 in | 698 |
| 245/70R17 | 30.5 in (775 mm) | 6.8 in | 661 |
| 265/70R17 | 31.6 in (803 mm) | 7.3 in | 638 |
| 275/65R18 | 32.1 in (815 mm) | 7.0 in | 629 |
| 275/60R20 | 33.0 in (838 mm) | 6.5 in | 611 |
| 285/70R17 | 32.7 in (831 mm) | 7.9 in | 617 |
| 285/75R17 | 33.8 in (859 mm) | 8.4 in | 596 |
| 315/70R17 | 34.4 in (873 mm) | 8.7 in | 587 |
| 275/70R18 | 33.2 in (842 mm) | 7.6 in | 608 |
| 225/75R15 | 28.3 in (719 mm) | 6.6 in | 713 |
Calculated sizes. Real tires vary slightly by brand and tread depth. The verdict is “OK” within ±3% of the original diameter, “at the limit” up to ±5% and “exceeds” beyond that.
Tire size and towing
Tire diameter changes your effective gearing: taller tires reduce pulling power and engine RPM at a given speed. See how much with theeffective gear ratio calculator. Width and sidewall also matter when towing: a taller, softer sidewall can feel less stable with a heavy tongue load. Above all, the load index must cover your axle loads; for trailers, check the load per tire with the trailer tire load calculator.
Common mistakes
- Comparing widths only. Aspect ratio and rim size change the diameter far more.
- Ignoring load rating. A bigger tire is not necessarily a stronger one.
- Mixing sizes on an axle. Keep both tires on an axle the same size and type.
- Forgetting clearance. Check full steering lock and suspension travel for rubbing.