285/60R18 tyre size
A 285/60R18 tyre is 285 mm (11.2") wide with a sidewall height of 60 % of the width, i.e. 171.0 mm (6.7"), on a 18" (457 mm) rim. Overall diameter is 799.2 mm (31.5"), so the closest inch-style size is 31.5×11.2R18. Static circumference is 2511 mm; the ETRTO rolling circumference is 2438 mm, giving 410 revolutions per km (660 per mile).
Dimensions and rolling diameter
| Section width | 285 mm | 11.22″ |
| Sidewall height | 171.0 mm | 6.73″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 799.2 mm | 31.46″ |
| Static radius | 399.6 mm | 15.73″ |
| Static circumference (π × D) | 2,511 mm | 98.8″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,438 mm | 96.0″ |
| Effective rolling radius | 387.9 mm | 15.27″ |
| Revs per km (rolling / static) | 410 / 398 | |
| Inch equivalent | 31.5×11.2R18 | |
| Approved rim widths (approx.) | 8″–9.5″ (measuring rim 8.5″) | |
A 285/60R18 tyre is 285 mm (11.2") wide with a sidewall height of 60 % of the width, i.e. 171.0 mm (6.7"), on a 18" (457 mm) rim. Overall diameter is 799.2 mm (31.5"), so the closest inch-style size is 31.5×11.2R18. Static circumference is 2511 mm; the ETRTO rolling circumference is 2438 mm, giving 410 revolutions per km (660 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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