235/40R18 tyre size
A 235/40R18 tyre is 235 mm (9.3") wide with a sidewall height of 40 % of the width, i.e. 94.0 mm (3.7"), on a 18" (457 mm) rim. Overall diameter is 645.2 mm (25.4"), so the closest inch-style size is 25.4×9.3R18. Static circumference is 2027 mm; the ETRTO rolling circumference is 1968 mm, giving 508 revolutions per km (818 per mile).
Dimensions and rolling diameter
| Section width | 235 mm | 9.25″ |
| Sidewall height | 94.0 mm | 3.70″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 645.2 mm | 25.40″ |
| Static radius | 322.6 mm | 12.70″ |
| Static circumference (π × D) | 2,027 mm | 79.8″ |
| Rolling circumference (ETRTO 3.05 × D) | 1,968 mm | 77.5″ |
| Effective rolling radius | 313.2 mm | 12.33″ |
| Revs per km (rolling / static) | 508 / 493 | |
| Inch equivalent | 25.4×9.3R18 | |
| Approved rim widths (approx.) | 8″–9.5″ (measuring rim 8.5″) | |
A 235/40R18 tyre is 235 mm (9.3") wide with a sidewall height of 40 % of the width, i.e. 94.0 mm (3.7"), on a 18" (457 mm) rim. Overall diameter is 645.2 mm (25.4"), so the closest inch-style size is 25.4×9.3R18. Static circumference is 2027 mm; the ETRTO rolling circumference is 1968 mm, giving 508 revolutions per km (818 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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