235/45R18 tyre size
A 235/45R18 tyre is 235 mm (9.3") wide with a sidewall height of 45 % of the width, i.e. 105.8 mm (4.2"), on a 18" (457 mm) rim. Overall diameter is 668.7 mm (26.3"), so the closest inch-style size is 26.3×9.3R18. Static circumference is 2101 mm; the ETRTO rolling circumference is 2040 mm, giving 490 revolutions per km (789 per mile).
Dimensions and rolling diameter
| Section width | 235 mm | 9.25″ |
| Sidewall height | 105.8 mm | 4.16″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 668.7 mm | 26.33″ |
| Static radius | 334.4 mm | 13.16″ |
| Static circumference (π × D) | 2,101 mm | 82.7″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,040 mm | 80.3″ |
| Effective rolling radius | 324.6 mm | 12.78″ |
| Revs per km (rolling / static) | 490 / 476 | |
| Inch equivalent | 26.3×9.3R18 | |
| Approved rim widths (approx.) | 7.5″–9″ (measuring rim 8″) | |
A 235/45R18 tyre is 235 mm (9.3") wide with a sidewall height of 45 % of the width, i.e. 105.8 mm (4.2"), on a 18" (457 mm) rim. Overall diameter is 668.7 mm (26.3"), so the closest inch-style size is 26.3×9.3R18. Static circumference is 2101 mm; the ETRTO rolling circumference is 2040 mm, giving 490 revolutions per km (789 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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