ElpiAuto235/40R18 vs 255/40R18
255/40R18 is 0.63″ (16 mm) taller than 235/40R18, a 2.5% 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 | 235/40R18 | 255/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 25.40″ | 26.03″ | +0.63″ | +2.5% |
| Section width | 9.25″ | 10.04″ | +0.79″ | +8.5% |
| Sidewall height | 3.70″ | 4.02″ | +0.31″ | +8.5% |
| Circumference | 79.80″ | 81.78″ | +1.98″ | +2.5% |
| Revs per mile | 794.0 | 774.8 | −19.2 | −2.4% |
| Revs per km | 493.4 | 481.4 | −11.9 | −2.4% |
Speedometer: with 235/40R18 as the original, 255/40R18 makes an indicated 60 mph really about 61.5 mph, and the axle sits ~0.31″ 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.