ElpiAuto245/40R18 vs 255/40R18
255/40R18 is 0.31″ (8 mm) taller than 245/40R18, a 1.2% difference in overall diameter — comfortably inside the ±3% band most fitment guides use, so it swaps straight across for gearing and the speedometer.
| Measurement | 245/40R18 | 255/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 25.72″ | 26.03″ | +0.31″ | +1.2% |
| Section width | 9.65″ | 10.04″ | +0.39″ | +4.1% |
| Sidewall height | 3.86″ | 4.02″ | +0.16″ | +4.1% |
| Circumference | 80.79″ | 81.78″ | +0.99″ | +1.2% |
| Revs per mile | 784.2 | 774.8 | −9.5 | −1.2% |
| Revs per km | 487.3 | 481.4 | −5.9 | −1.2% |
Speedometer: with 245/40R18 as the original, 255/40R18 makes an indicated 60 mph really about 60.7 mph, and the axle sits ~0.16″ 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.