ElpiAuto235/40R18 vs 255/35R18
255/35R18 is 0.37″ (10 mm) shorter than 235/40R18, a 1.5% difference in overall diameter — comfortably inside the ±3% band most fitment guides use, so it swaps straight across for gearing and the speedometer.
| Measurement | 235/40R18 | 255/35R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 25.40″ | 25.03″ | −0.37″ | −1.5% |
| Section width | 9.25″ | 10.04″ | +0.79″ | +8.5% |
| Sidewall height | 3.70″ | 3.51″ | −0.19″ | −5.1% |
| Circumference | 79.80″ | 78.63″ | −1.18″ | −1.5% |
| Revs per mile | 794.0 | 805.8 | +11.9 | +1.5% |
| Revs per km | 493.4 | 500.7 | +7.4 | +1.5% |
Speedometer: with 235/40R18 as the original, 255/35R18 makes an indicated 60 mph really about 59.1 mph, and the axle sits ~0.19″ lower.
Reading this comparison
The overall diameter is what matters here: 235/40R18 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.