ElpiAuto225/40R18 vs 255/40R18
255/40R18 is 0.94″ (24 mm) taller than 225/40R18, a 3.8% difference in overall diameter — outside the ±3% band, so expect the speedometer to drift and check for rubbing before fitting.
| Measurement | 225/40R18 | 255/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 25.09″ | 26.03″ | +0.94″ | +3.8% |
| Section width | 8.86″ | 10.04″ | +1.18″ | +13.3% |
| Sidewall height | 3.54″ | 4.02″ | +0.47″ | +13.3% |
| Circumference | 78.81″ | 81.78″ | +2.97″ | +3.8% |
| Revs per mile | 803.9 | 774.8 | −29.2 | −3.6% |
| Revs per km | 499.5 | 481.4 | −18.1 | −3.6% |
Speedometer: with 225/40R18 as the original, 255/40R18 makes an indicated 60 mph really about 62.3 mph, and the axle sits ~0.47″ higher.
Reading this comparison
The overall diameter is what matters here: 255/40R18 is the taller tire. It sets ride height, gearing and how far the tire travels per revolution — which is what the speedometer and odometer count. Width and sidewall change fit and feel (a wider tire needs a wider wheel and more clearance; a shorter sidewall sharpens steering) but not the gearing.
Figures are geometric estimates from the size codes; real moulded tires vary by brand by 1–3%. Confirm load rating, wheel width and physical clearance on your vehicle before fitting. To compare any other two sizes, use the tire size comparison tool; to decode one size and see what else fits its wheel, the tire size calculator.