265/50R20 tyre size
A 265/50R20 tyre is 265 mm (10.4") wide with a sidewall height of 50 % of the width, i.e. 132.5 mm (5.2"), on a 20" (508 mm) rim. Overall diameter is 773.0 mm (30.4"), so the closest inch-style size is 30.4×10.4R20. Static circumference is 2428 mm; the ETRTO rolling circumference is 2358 mm, giving 424 revolutions per km (683 per mile).
Dimensions and rolling diameter
| Section width | 265 mm | 10.43″ |
| Sidewall height | 132.5 mm | 5.22″ |
| Rim diameter | 508.0 mm | 20″ |
| Overall diameter (static, unloaded) | 773.0 mm | 30.43″ |
| Static radius | 386.5 mm | 15.22″ |
| Static circumference (π × D) | 2,428 mm | 95.6″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,358 mm | 92.8″ |
| Effective rolling radius | 375.2 mm | 14.77″ |
| Revs per km (rolling / static) | 424 / 412 | |
| Inch equivalent | 30.4×10.4R20 | |
| Approved rim widths (approx.) | 7.5″–9.5″ (measuring rim 8.5″) | |
A 265/50R20 tyre is 265 mm (10.4") wide with a sidewall height of 50 % of the width, i.e. 132.5 mm (5.2"), on a 20" (508 mm) rim. Overall diameter is 773.0 mm (30.4"), so the closest inch-style size is 30.4×10.4R20. Static circumference is 2428 mm; the ETRTO rolling circumference is 2358 mm, giving 424 revolutions per km (683 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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