255/60R18 tyre size
A 255/60R18 tyre is 255 mm (10.0") wide with a sidewall height of 60 % of the width, i.e. 153.0 mm (6.0"), on a 18" (457 mm) rim. Overall diameter is 763.2 mm (30.0"), so the closest inch-style size is 30x10R18. Static circumference is 2398 mm; the ETRTO rolling circumference is 2328 mm, giving 430 revolutions per km (691 per mile).
Dimensions and rolling diameter
| Section width | 255 mm | 10.04″ |
| Sidewall height | 153.0 mm | 6.02″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 763.2 mm | 30.05″ |
| Static radius | 381.6 mm | 15.02″ |
| Static circumference (π × D) | 2,398 mm | 94.4″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,328 mm | 91.6″ |
| Effective rolling radius | 370.5 mm | 14.59″ |
| Revs per km (rolling / static) | 430 / 417 | |
| Inch equivalent | 30x10R18 | |
| Approved rim widths (approx.) | 7″–8.5″ (measuring rim 7.5″) | |
A 255/60R18 tyre is 255 mm (10.0") wide with a sidewall height of 60 % of the width, i.e. 153.0 mm (6.0"), on a 18" (457 mm) rim. Overall diameter is 763.2 mm (30.0"), so the closest inch-style size is 30x10R18. Static circumference is 2398 mm; the ETRTO rolling circumference is 2328 mm, giving 430 revolutions per km (691 per mile).
Why two circumferences? The static figure is simple geometry (π × diameter). Under load a radial tyre flattens and rolls about 3 % less per revolution; ETRTO defines the rolling circumference as 3.05 × the nominal diameter, which is what manufacturers quote and what speedo and gearing calculations should use.
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