ElpiAuto245/40R18 vs 255/35R18
255/35R18 is 0.69″ (17 mm) shorter than 245/40R18, a 2.7% difference in overall diameter — right at the edge of the ±3% band — fine on most cars, but check clearance at full lock and full suspension compression.
| Measurement | 245/40R18 | 255/35R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 25.72″ | 25.03″ | −0.69″ | −2.7% |
| Section width | 9.65″ | 10.04″ | +0.39″ | +4.1% |
| Sidewall height | 3.86″ | 3.51″ | −0.34″ | −8.9% |
| Circumference | 80.79″ | 78.63″ | −2.16″ | −2.7% |
| Revs per mile | 784.2 | 805.8 | +21.6 | +2.8% |
| Revs per km | 487.3 | 500.7 | +13.4 | +2.8% |
Speedometer: with 245/40R18 as the original, 255/35R18 makes an indicated 60 mph really about 58.4 mph, and the axle sits ~0.34″ lower.
Reading this comparison
The overall diameter is what matters here: 245/40R18 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.