31×10.5R15 tyre size
A 31×10.5R15 tyre is 267 mm (10.5") wide with a sidewall height of 76 % of the width, i.e. 203.2 mm (8.0"), on a 15" (381 mm) rim. Overall diameter is 787.4 mm (31.0"), so the closest inch-style size is 31×10.5R15. Static circumference is 2474 mm; the ETRTO rolling circumference is 2402 mm, giving 416 revolutions per km (670 per mile).
Dimensions and rolling diameter
| Section width | 267 mm | 10.50″ |
| Sidewall height | 203.2 mm | 8.00″ |
| Rim diameter | 381.0 mm | 15″ |
| Overall diameter (static, unloaded) | 787.4 mm | 31.00″ |
| Static radius | 393.7 mm | 15.50″ |
| Static circumference (π × D) | 2,474 mm | 97.4″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,402 mm | 94.6″ |
| Effective rolling radius | 382.2 mm | 15.05″ |
| Revs per km (rolling / static) | 416 / 404 | |
| Inch equivalent | 31×10.5R15 · metric equivalent 265/75R15 | |
| Approved rim widths (approx.) | 7″–8.5″ (measuring rim 7.5″) | |
A 31×10.5R15 tyre is 267 mm (10.5") wide with a sidewall height of 76 % of the width, i.e. 203.2 mm (8.0"), on a 15" (381 mm) rim. Overall diameter is 787.4 mm (31.0"), so the closest inch-style size is 31×10.5R15. Static circumference is 2474 mm; the ETRTO rolling circumference is 2402 mm, giving 416 revolutions per km (670 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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