265/65R18 tyre size
A 265/65R18 tyre is 265 mm (10.4") wide with a sidewall height of 65 % of the width, i.e. 172.3 mm (6.8"), on a 18" (457 mm) rim. Overall diameter is 801.7 mm (31.6"), so the closest inch-style size is 31.6×10.4R18. Static circumference is 2519 mm; the ETRTO rolling circumference is 2445 mm, giving 409 revolutions per km (658 per mile).
Dimensions and rolling diameter
| Section width | 265 mm | 10.43″ |
| Sidewall height | 172.3 mm | 6.78″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 801.7 mm | 31.56″ |
| Static radius | 400.9 mm | 15.78″ |
| Static circumference (π × D) | 2,519 mm | 99.2″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,445 mm | 96.3″ |
| Effective rolling radius | 389.2 mm | 15.32″ |
| Revs per km (rolling / static) | 409 / 397 | |
| Inch equivalent | 31.6×10.4R18 | |
| Approved rim widths (approx.) | 7.5″–9″ (measuring rim 8″) | |
A 265/65R18 tyre is 265 mm (10.4") wide with a sidewall height of 65 % of the width, i.e. 172.3 mm (6.8"), on a 18" (457 mm) rim. Overall diameter is 801.7 mm (31.6"), so the closest inch-style size is 31.6×10.4R18. Static circumference is 2519 mm; the ETRTO rolling circumference is 2445 mm, giving 409 revolutions per km (658 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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