275/50R20 tyre size
A 275/50R20 tyre is 275 mm (10.8") wide with a sidewall height of 50 % of the width, i.e. 137.5 mm (5.4"), on a 20" (508 mm) rim. Overall diameter is 783.0 mm (30.8"), so the closest inch-style size is 30.8×10.8R20. Static circumference is 2460 mm; the ETRTO rolling circumference is 2388 mm, giving 419 revolutions per km (674 per mile).
Dimensions and rolling diameter
| Section width | 275 mm | 10.83″ |
| Sidewall height | 137.5 mm | 5.41″ |
| Rim diameter | 508.0 mm | 20″ |
| Overall diameter (static, unloaded) | 783.0 mm | 30.83″ |
| Static radius | 391.5 mm | 15.41″ |
| Static circumference (π × D) | 2,460 mm | 96.8″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,388 mm | 94.0″ |
| Effective rolling radius | 380.1 mm | 14.96″ |
| Revs per km (rolling / static) | 419 / 407 | |
| Inch equivalent | 30.8×10.8R20 | |
| Approved rim widths (approx.) | 7.5″–9.5″ (measuring rim 8.5″) | |
A 275/50R20 tyre is 275 mm (10.8") wide with a sidewall height of 50 % of the width, i.e. 137.5 mm (5.4"), on a 20" (508 mm) rim. Overall diameter is 783.0 mm (30.8"), so the closest inch-style size is 30.8×10.8R20. Static circumference is 2460 mm; the ETRTO rolling circumference is 2388 mm, giving 419 revolutions per km (674 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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