245/45R18 tyre size
A 245/45R18 tyre is 245 mm (9.6") wide with a sidewall height of 45 % of the width, i.e. 110.3 mm (4.3"), on a 18" (457 mm) rim. Overall diameter is 677.7 mm (26.7"), so the closest inch-style size is 26.7×9.6R18. Static circumference is 2129 mm; the ETRTO rolling circumference is 2067 mm, giving 484 revolutions per km (779 per mile).
Dimensions and rolling diameter
| Section width | 245 mm | 9.65″ |
| Sidewall height | 110.3 mm | 4.34″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 677.7 mm | 26.68″ |
| Static radius | 338.9 mm | 13.34″ |
| Static circumference (π × D) | 2,129 mm | 83.8″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,067 mm | 81.4″ |
| Effective rolling radius | 329.0 mm | 12.95″ |
| Revs per km (rolling / static) | 484 / 470 | |
| Inch equivalent | 26.7×9.6R18 | |
| Approved rim widths (approx.) | 7.5″–9″ (measuring rim 8″) | |
A 245/45R18 tyre is 245 mm (9.6") wide with a sidewall height of 45 % of the width, i.e. 110.3 mm (4.3"), on a 18" (457 mm) rim. Overall diameter is 677.7 mm (26.7"), so the closest inch-style size is 26.7×9.6R18. Static circumference is 2129 mm; the ETRTO rolling circumference is 2067 mm, giving 484 revolutions per km (779 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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