235/55R18 tyre size
A 235/55R18 tyre is 235 mm (9.3") wide with a sidewall height of 55 % of the width, i.e. 129.3 mm (5.1"), on a 18" (457 mm) rim. Overall diameter is 715.7 mm (28.2"), so the closest inch-style size is 28.2×9.3R18. Static circumference is 2248 mm; the ETRTO rolling circumference is 2183 mm, giving 458 revolutions per km (737 per mile).
Dimensions and rolling diameter
| Section width | 235 mm | 9.25″ |
| Sidewall height | 129.3 mm | 5.09″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 715.7 mm | 28.18″ |
| Static radius | 357.9 mm | 14.09″ |
| Static circumference (π × D) | 2,248 mm | 88.5″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,183 mm | 85.9″ |
| Effective rolling radius | 347.4 mm | 13.68″ |
| Revs per km (rolling / static) | 458 / 445 | |
| Inch equivalent | 28.2×9.3R18 | |
| Approved rim widths (approx.) | 6.5″–8.5″ (measuring rim 7.5″) | |
A 235/55R18 tyre is 235 mm (9.3") wide with a sidewall height of 55 % of the width, i.e. 129.3 mm (5.1"), on a 18" (457 mm) rim. Overall diameter is 715.7 mm (28.2"), so the closest inch-style size is 28.2×9.3R18. Static circumference is 2248 mm; the ETRTO rolling circumference is 2183 mm, giving 458 revolutions per km (737 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.
Buying tyres for your business?
Onyx Tyres supplies workshops, retailers and fleets across Australia at wholesale prices. Open a trade account to see stock and pricing and order online.