245/40R18 tyre size
A 245/40R18 tyre is 245 mm (9.6") wide with a sidewall height of 40 % of the width, i.e. 98.0 mm (3.9"), on a 18" (457 mm) rim. Overall diameter is 653.2 mm (25.7"), so the closest inch-style size is 25.7×9.6R18. Static circumference is 2052 mm; the ETRTO rolling circumference is 1992 mm, giving 502 revolutions per km (808 per mile).
Dimensions and rolling diameter
| Section width | 245 mm | 9.65″ |
| Sidewall height | 98.0 mm | 3.86″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 653.2 mm | 25.72″ |
| Static radius | 326.6 mm | 12.86″ |
| Static circumference (π × D) | 2,052 mm | 80.8″ |
| Rolling circumference (ETRTO 3.05 × D) | 1,992 mm | 78.4″ |
| Effective rolling radius | 317.1 mm | 12.48″ |
| Revs per km (rolling / static) | 502 / 487 | |
| Inch equivalent | 25.7×9.6R18 | |
| Approved rim widths (approx.) | 8″–9.5″ (measuring rim 8.5″) | |
A 245/40R18 tyre is 245 mm (9.6") wide with a sidewall height of 40 % of the width, i.e. 98.0 mm (3.9"), on a 18" (457 mm) rim. Overall diameter is 653.2 mm (25.7"), so the closest inch-style size is 25.7×9.6R18. Static circumference is 2052 mm; the ETRTO rolling circumference is 1992 mm, giving 502 revolutions per km (808 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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