255/35R18 tyre size
A 255/35R18 tyre is 255 mm (10.0") wide with a sidewall height of 35 % of the width, i.e. 89.3 mm (3.5"), on a 18" (457 mm) rim. Overall diameter is 635.7 mm (25.0"), so the closest inch-style size is 25x10R18. Static circumference is 1997 mm; the ETRTO rolling circumference is 1939 mm, giving 516 revolutions per km (830 per mile).
Dimensions and rolling diameter
| Section width | 255 mm | 10.04″ |
| Sidewall height | 89.3 mm | 3.51″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 635.7 mm | 25.03″ |
| Static radius | 317.9 mm | 12.51″ |
| Static circumference (π × D) | 1,997 mm | 78.6″ |
| Rolling circumference (ETRTO 3.05 × D) | 1,939 mm | 76.3″ |
| Effective rolling radius | 308.6 mm | 12.15″ |
| Revs per km (rolling / static) | 516 / 501 | |
| Inch equivalent | 25x10R18 | |
| Approved rim widths (approx.) | 8.5″–10″ (measuring rim 9″) | |
A 255/35R18 tyre is 255 mm (10.0") wide with a sidewall height of 35 % of the width, i.e. 89.3 mm (3.5"), on a 18" (457 mm) rim. Overall diameter is 635.7 mm (25.0"), so the closest inch-style size is 25x10R18. Static circumference is 1997 mm; the ETRTO rolling circumference is 1939 mm, giving 516 revolutions per km (830 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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