225/45R18 tyre size
A 225/45R18 tyre is 225 mm (8.9") wide with a sidewall height of 45 % of the width, i.e. 101.3 mm (4.0"), on a 18" (457 mm) rim. Overall diameter is 659.7 mm (26.0"), so the closest inch-style size is 26×8.9R18. Static circumference is 2073 mm; the ETRTO rolling circumference is 2012 mm, giving 497 revolutions per km (800 per mile).
Dimensions and rolling diameter
| Section width | 225 mm | 8.86″ |
| Sidewall height | 101.3 mm | 3.99″ |
| Rim diameter | 457.2 mm | 18″ |
| Overall diameter (static, unloaded) | 659.7 mm | 25.97″ |
| Static radius | 329.9 mm | 12.99″ |
| Static circumference (π × D) | 2,073 mm | 81.6″ |
| Rolling circumference (ETRTO 3.05 × D) | 2,012 mm | 79.2″ |
| Effective rolling radius | 320.2 mm | 12.61″ |
| Revs per km (rolling / static) | 497 / 483 | |
| Inch equivalent | 26×8.9R18 | |
| Approved rim widths (approx.) | 7″–8.5″ (measuring rim 7.5″) | |
A 225/45R18 tyre is 225 mm (8.9") wide with a sidewall height of 45 % of the width, i.e. 101.3 mm (4.0"), on a 18" (457 mm) rim. Overall diameter is 659.7 mm (26.0"), so the closest inch-style size is 26×8.9R18. Static circumference is 2073 mm; the ETRTO rolling circumference is 2012 mm, giving 497 revolutions per km (800 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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