255/40R18 tyre size
A 255/40R18 tyre is 255 mm (10.0") wide with a sidewall height of 40 % of the width, i.e. 102.0 mm (4.0"), on a 18" (457 mm) rim. Overall diameter is 661.2 mm (26.0"), so the closest inch-style size is 26x10R18. Static circumference is 2077 mm; the ETRTO rolling circumference is 2017 mm, giving 496 revolutions per km (798 per mile).
Dimensions and rolling diameter
| Section width | 255 mm | 10.04″ |
| Sidewall height | 102.0 mm | 4.02″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 661.2 mm | 26.03″ |
| Static radius | 330.6 mm | 13.02″ |
| Static circumference (π × D) | 2,077 mm | 81.8″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,017 mm | 79.4″ |
| Effective rolling radius | 321.0 mm | 12.64″ |
| Revs per km (rolling / static) | 496 / 481 | |
| Inch equivalent | 26x10R18 | |
| Approved rim widths (approx.) | 8.5″–10″ (measuring rim 9″) | |
A 255/40R18 tyre is 255 mm (10.0") wide with a sidewall height of 40 % of the width, i.e. 102.0 mm (4.0"), on a 18" (457 mm) rim. Overall diameter is 661.2 mm (26.0"), so the closest inch-style size is 26x10R18. Static circumference is 2077 mm; the ETRTO rolling circumference is 2017 mm, giving 496 revolutions per km (798 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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