33×12.5R15 tyre size
A 33×12.5R15 tyre is 318 mm (12.5") wide with a sidewall height of 72 % of the width, i.e. 228.6 mm (9.0"), on a 15" (381 mm) rim. Overall diameter is 838.2 mm (33.0"), so the closest inch-style size is 33×12.5R15. Static circumference is 2633 mm; the ETRTO rolling circumference is 2557 mm, giving 391 revolutions per km (630 per mile).
Dimensions and rolling diameter
| Section width | 318 mm | 12.50″ |
| Sidewall height | 228.6 mm | 9.00″ |
| Rim diameter | 381.0 mm | 15″ |
| Overall diameter (static, unloaded) | 838.2 mm | 33.00″ |
| Static radius | 419.1 mm | 16.50″ |
| Static circumference (π × D) | 2,633 mm | 103.7″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,557 mm | 100.6″ |
| Effective rolling radius | 406.9 mm | 16.02″ |
| Revs per km (rolling / static) | 391 / 380 | |
| Inch equivalent | 33×12.5R15 · metric equivalent 315/75R15 | |
| Approved rim widths (approx.) | 9″–10.5″ (measuring rim 9.5″) | |
A 33×12.5R15 tyre is 318 mm (12.5") wide with a sidewall height of 72 % of the width, i.e. 228.6 mm (9.0"), on a 15" (381 mm) rim. Overall diameter is 838.2 mm (33.0"), so the closest inch-style size is 33×12.5R15. Static circumference is 2633 mm; the ETRTO rolling circumference is 2557 mm, giving 391 revolutions per km (630 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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