ElpiAuto225/40R18 vs 255/35R18
255/35R18 is 0.06″ (2 mm) shorter than 225/40R18, a 0.2% difference in overall diameter — comfortably inside the ±3% band most fitment guides use, so it swaps straight across for gearing and the speedometer.
| Measurement | 225/40R18 | 255/35R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 25.09″ | 25.03″ | −0.06″ | −0.2% |
| Section width | 8.86″ | 10.04″ | +1.18″ | +13.3% |
| Sidewall height | 3.54″ | 3.51″ | −0.03″ | −0.8% |
| Circumference | 78.81″ | 78.63″ | −0.19″ | −0.2% |
| Revs per mile | 803.9 | 805.8 | +1.9 | +0.2% |
| Revs per km | 499.5 | 500.7 | +1.2 | +0.2% |
Speedometer: with 225/40R18 as the original, 255/35R18 makes an indicated 60 mph really about 59.9 mph, and the axle sits ~0.03″ lower.
Reading this comparison
The overall diameter is what matters here: 225/40R18 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.