255/55R18 tyre size
A 255/55R18 tyre is 255 mm (10.0") wide with a sidewall height of 55 % of the width, i.e. 140.3 mm (5.5"), on a 18" (457 mm) rim. Overall diameter is 737.7 mm (29.0"), so the closest inch-style size is 29x10R18. Static circumference is 2318 mm; the ETRTO rolling circumference is 2250 mm, giving 444 revolutions per km (715 per mile).
Dimensions and rolling diameter
| Section width | 255 mm | 10.04″ |
| Sidewall height | 140.3 mm | 5.52″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 737.7 mm | 29.04″ |
| Static radius | 368.9 mm | 14.52″ |
| Static circumference (π × D) | 2,318 mm | 91.2″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,250 mm | 88.6″ |
| Effective rolling radius | 358.1 mm | 14.10″ |
| Revs per km (rolling / static) | 444 / 431 | |
| Inch equivalent | 29x10R18 | |
| Approved rim widths (approx.) | 7″–9″ (measuring rim 8″) | |
A 255/55R18 tyre is 255 mm (10.0") wide with a sidewall height of 55 % of the width, i.e. 140.3 mm (5.5"), on a 18" (457 mm) rim. Overall diameter is 737.7 mm (29.0"), so the closest inch-style size is 29x10R18. Static circumference is 2318 mm; the ETRTO rolling circumference is 2250 mm, giving 444 revolutions per km (715 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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