275/65R18 tyre size
A 275/65R18 tyre is 275 mm (10.8") wide with a sidewall height of 65 % of the width, i.e. 178.8 mm (7.0"), on a 18" (457 mm) rim. Overall diameter is 814.7 mm (32.1"), so the closest inch-style size is 32.1×10.8R18. Static circumference is 2559 mm; the ETRTO rolling circumference is 2485 mm, giving 402 revolutions per km (648 per mile).
Dimensions and rolling diameter
| Section width | 275 mm | 10.83″ |
| Sidewall height | 178.8 mm | 7.04″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 814.7 mm | 32.07″ |
| Static radius | 407.4 mm | 16.04″ |
| Static circumference (π × D) | 2,559 mm | 100.8″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,485 mm | 97.8″ |
| Effective rolling radius | 395.5 mm | 15.57″ |
| Revs per km (rolling / static) | 402 / 391 | |
| Inch equivalent | 32.1×10.8R18 | |
| Approved rim widths (approx.) | 7.5″–9″ (measuring rim 8″) | |
A 275/65R18 tyre is 275 mm (10.8") wide with a sidewall height of 65 % of the width, i.e. 178.8 mm (7.0"), on a 18" (457 mm) rim. Overall diameter is 814.7 mm (32.1"), so the closest inch-style size is 32.1×10.8R18. Static circumference is 2559 mm; the ETRTO rolling circumference is 2485 mm, giving 402 revolutions per km (648 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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