ElpiAuto235/40R18 vs 245/45R18
245/45R18 is 1.28″ (33 mm) taller than 235/40R18, a 5.0% difference in overall diameter — outside the ±3% band, so expect the speedometer to drift and check for rubbing before fitting.
| Measurement | 235/40R18 | 245/45R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 25.40″ | 26.68″ | +1.28″ | +5.0% |
| Section width | 9.25″ | 9.65″ | +0.39″ | +4.3% |
| Sidewall height | 3.70″ | 4.34″ | +0.64″ | +17.3% |
| Circumference | 79.80″ | 83.82″ | +4.02″ | +5.0% |
| Revs per mile | 794.0 | 755.9 | −38.1 | −4.8% |
| Revs per km | 493.4 | 469.7 | −23.7 | −4.8% |
Speedometer: with 235/40R18 as the original, 245/45R18 makes an indicated 60 mph really about 63.0 mph, and the axle sits ~0.64″ higher.
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.