ElpiAuto225/40R18 vs 235/40R18
235/40R18 is 0.31″ (8 mm) taller than 225/40R18, a 1.3% difference in overall diameter — comfortably inside the ±3% band most fitment guides use, so it swaps straight across for gearing and the speedometer.
| Measurement | 225/40R18 | 235/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 25.09″ | 25.40″ | +0.31″ | +1.3% |
| Section width | 8.86″ | 9.25″ | +0.39″ | +4.4% |
| Sidewall height | 3.54″ | 3.70″ | +0.16″ | +4.4% |
| Circumference | 78.81″ | 79.80″ | +0.99″ | +1.3% |
| Revs per mile | 803.9 | 794.0 | −10.0 | −1.2% |
| Revs per km | 499.5 | 493.4 | −6.2 | −1.2% |
Speedometer: with 225/40R18 as the original, 235/40R18 makes an indicated 60 mph really about 60.8 mph, and the axle sits ~0.16″ higher.
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.