275/55R20 tyre size
A 275/55R20 tyre is 275 mm (10.8") wide with a sidewall height of 55 % of the width, i.e. 151.3 mm (6.0"), on a 20" (508 mm) rim. Overall diameter is 810.5 mm (31.9"), so the closest inch-style size is 31.9×10.8R20. Static circumference is 2546 mm; the ETRTO rolling circumference is 2472 mm, giving 405 revolutions per km (651 per mile).
Dimensions and rolling diameter
| Section width | 275 mm | 10.83″ |
| Sidewall height | 151.3 mm | 5.95″ |
| Rim diameter | 508.0 mm | 20″ |
| Overall diameter (static, unloaded) | 810.5 mm | 31.91″ |
| Static radius | 405.3 mm | 15.95″ |
| Static circumference (π × D) | 2,546 mm | 100.2″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,472 mm | 97.3″ |
| Effective rolling radius | 393.4 mm | 15.49″ |
| Revs per km (rolling / static) | 405 / 393 | |
| Inch equivalent | 31.9×10.8R20 | |
| Approved rim widths (approx.) | 7.5″–9.5″ (measuring rim 8.5″) | |
A 275/55R20 tyre is 275 mm (10.8") wide with a sidewall height of 55 % of the width, i.e. 151.3 mm (6.0"), on a 20" (508 mm) rim. Overall diameter is 810.5 mm (31.9"), so the closest inch-style size is 31.9×10.8R20. Static circumference is 2546 mm; the ETRTO rolling circumference is 2472 mm, giving 405 revolutions per km (651 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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