225/40R18 Tyre Size — Dimensions & Alternatives
Get dimensions for 225/40R18 tires and compatible alternatives.
225/40R18 dimensions
| Section width | 225mm |
| Aspect ratio | 40% |
| Sidewall height | 90mm (3.54in) |
| Rim diameter | 18" (457.2mm) |
| Overall diameter | 637mm (25.08in) |
| Circumference | 2001mm |
| Revs per km | 500 |
What does 225/40R18 mean?
The tyre designation 225/40R18 breaks down into three parts: the section width (225mm — how wide the tread is), the aspect ratio (40% — sidewall height as a percentage of width), and the rim diameter (18 inches). The "R" indicates radial construction, which is standard on nearly every passenger vehicle today.
Can I fit an alternative size?
Yes — tyres with a slightly different section width, aspect ratio or rim size can often be fitted safely provided the overall diameter stays within about 3% of the original. Going wider improves grip but increases rolling resistance; going narrower reduces road noise and fuel consumption. The lookup tool below shows compatible alternative sizes for 225/40R18.
Speedometer error
Changing tyre size affects your speedometer reading. A larger overall diameter means the speedometer under-reads (you're actually going faster than the needle shows); a smaller diameter means it over-reads. Stay within 3% to keep the error under the legal tolerance in Australia and New Zealand.
