ElpiAuto245/45R18 vs 255/40R18
255/40R18 is 0.65″ (17 mm) shorter than 245/45R18, a 2.4% 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 | 245/45R18 | 255/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 26.68″ | 26.03″ | −0.65″ | −2.4% |
| Section width | 9.65″ | 10.04″ | +0.39″ | +4.1% |
| Sidewall height | 4.34″ | 4.02″ | −0.32″ | −7.5% |
| Circumference | 83.82″ | 81.78″ | −2.04″ | −2.4% |
| Revs per mile | 755.9 | 774.8 | +18.9 | +2.5% |
| Revs per km | 469.7 | 481.4 | +11.7 | +2.5% |
Speedometer: with 245/45R18 as the original, 255/40R18 makes an indicated 60 mph really about 58.5 mph, and the axle sits ~0.32″ lower.
Reading this comparison
The overall diameter is what matters here: 245/45R18 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.