225/40R18 tyre size
A 225/40R18 tyre is 225 mm (8.9") wide with a sidewall height of 40 % of the width, i.e. 90.0 mm (3.5"), on a 18" (457 mm) rim. Overall diameter is 637.2 mm (25.1"), so the closest inch-style size is 25.1×8.9R18. Static circumference is 2002 mm; the ETRTO rolling circumference is 1943 mm, giving 515 revolutions per km (828 per mile).
Dimensions and rolling diameter
| Section width | 225 mm | 8.86″ |
| Sidewall height | 90.0 mm | 3.54″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 637.2 mm | 25.09″ |
| Static radius | 318.6 mm | 12.54″ |
| Static circumference (π × D) | 2,002 mm | 78.8″ |
| Rolling circumference (ETRTO 3.05 × D) | 1,943 mm | 76.5″ |
| Effective rolling radius | 309.3 mm | 12.18″ |
| Revs per km (rolling / static) | 515 / 500 | |
| Inch equivalent | 25.1×8.9R18 | |
| Approved rim widths (approx.) | 7.5″–9″ (measuring rim 8″) | |
A 225/40R18 tyre is 225 mm (8.9") wide with a sidewall height of 40 % of the width, i.e. 90.0 mm (3.5"), on a 18" (457 mm) rim. Overall diameter is 637.2 mm (25.1"), so the closest inch-style size is 25.1×8.9R18. Static circumference is 2002 mm; the ETRTO rolling circumference is 1943 mm, giving 515 revolutions per km (828 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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