ElpiAuto255/35R18 vs 255/40R18
255/40R18 is 1.00″ (25 mm) taller than 255/35R18, a 4.0% difference in overall diameter — outside the ±3% band, so expect the speedometer to drift and check for rubbing before fitting.
| Measurement | 255/35R18 | 255/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 25.03″ | 26.03″ | +1.00″ | +4.0% |
| Section width | 10.04″ | 10.04″ | — | |
| Sidewall height | 3.51″ | 4.02″ | +0.50″ | +14.3% |
| Circumference | 78.63″ | 81.78″ | +3.15″ | +4.0% |
| Revs per mile | 805.8 | 774.8 | −31.1 | −3.9% |
| Revs per km | 500.7 | 481.4 | −19.3 | −3.9% |
Speedometer: with 255/35R18 as the original, 255/40R18 makes an indicated 60 mph really about 62.4 mph, and the axle sits ~0.50″ 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.