ElpiAuto235/45R18 vs 245/40R18
245/40R18 is 0.61″ (15 mm) shorter than 235/45R18, a 2.3% 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 | 235/45R18 | 245/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 26.33″ | 25.72″ | −0.61″ | −2.3% |
| Section width | 9.25″ | 9.65″ | +0.39″ | +4.3% |
| Sidewall height | 4.16″ | 3.86″ | −0.31″ | −7.3% |
| Circumference | 82.71″ | 80.79″ | −1.92″ | −2.3% |
| Revs per mile | 766.1 | 784.2 | +18.2 | +2.4% |
| Revs per km | 476.0 | 487.3 | +11.3 | +2.4% |
Speedometer: with 235/45R18 as the original, 245/40R18 makes an indicated 60 mph really about 58.6 mph, and the axle sits ~0.31″ lower.
Reading this comparison
The overall diameter is what matters here: 235/45R18 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.