235/50R18 tyre size
A 235/50R18 tyre is 235 mm (9.3") wide with a sidewall height of 50 % of the width, i.e. 117.5 mm (4.6"), on a 18" (457 mm) rim. Overall diameter is 692.2 mm (27.3"), so the closest inch-style size is 27.3×9.3R18. Static circumference is 2175 mm; the ETRTO rolling circumference is 2111 mm, giving 474 revolutions per km (762 per mile).
Dimensions and rolling diameter
| Section width | 235 mm | 9.25″ |
| Sidewall height | 117.5 mm | 4.63″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 692.2 mm | 27.25″ |
| Static radius | 346.1 mm | 13.63″ |
| Static circumference (π × D) | 2,175 mm | 85.6″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,111 mm | 83.1″ |
| Effective rolling radius | 336.0 mm | 13.23″ |
| Revs per km (rolling / static) | 474 / 460 | |
| Inch equivalent | 27.3×9.3R18 | |
| Approved rim widths (approx.) | 6.5″–8.5″ (measuring rim 7.5″) | |
A 235/50R18 tyre is 235 mm (9.3") wide with a sidewall height of 50 % of the width, i.e. 117.5 mm (4.6"), on a 18" (457 mm) rim. Overall diameter is 692.2 mm (27.3"), so the closest inch-style size is 27.3×9.3R18. Static circumference is 2175 mm; the ETRTO rolling circumference is 2111 mm, giving 474 revolutions per km (762 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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