275/45R20 tyre size
A 275/45R20 tyre is 275 mm (10.8") wide with a sidewall height of 45 % of the width, i.e. 123.8 mm (4.9"), on a 20" (508 mm) rim. Overall diameter is 755.5 mm (29.7"), so the closest inch-style size is 29.7×10.8R20. Static circumference is 2373 mm; the ETRTO rolling circumference is 2304 mm, giving 434 revolutions per km (698 per mile).
Dimensions and rolling diameter
| Section width | 275 mm | 10.83″ |
| Sidewall height | 123.8 mm | 4.87″ |
| Rim diameter | 508.0 mm | 20″ |
| Overall diameter (static, unloaded) | 755.5 mm | 29.74″ |
| Static radius | 377.8 mm | 14.87″ |
| Static circumference (π × D) | 2,373 mm | 93.4″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,304 mm | 90.7″ |
| Effective rolling radius | 366.7 mm | 14.44″ |
| Revs per km (rolling / static) | 434 / 421 | |
| Inch equivalent | 29.7×10.8R20 | |
| Approved rim widths (approx.) | 8.5″–10″ (measuring rim 9″) | |
A 275/45R20 tyre is 275 mm (10.8") wide with a sidewall height of 45 % of the width, i.e. 123.8 mm (4.9"), on a 20" (508 mm) rim. Overall diameter is 755.5 mm (29.7"), so the closest inch-style size is 29.7×10.8R20. Static circumference is 2373 mm; the ETRTO rolling circumference is 2304 mm, giving 434 revolutions per km (698 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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