ElpiAuto225/45R17 vs 245/40R18
245/40R18 is 0.74″ (19 mm) taller than 225/45R17, a 3.0% 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.
These sit on different wheel sizes — you'd be changing wheels too, not just tires.
| Measurement | 225/45R17 | 245/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 24.97″ | 25.72″ | +0.74″ | +3.0% |
| Section width | 8.86″ | 9.65″ | +0.79″ | +8.9% |
| Sidewall height | 3.99″ | 3.86″ | −0.13″ | −3.2% |
| Circumference | 78.45″ | 80.79″ | +2.34″ | +3.0% |
| Revs per mile | 807.6 | 784.2 | −23.4 | −2.9% |
| Revs per km | 501.8 | 487.3 | −14.5 | −2.9% |
Speedometer: with 225/45R17 as the original, 245/40R18 makes an indicated 60 mph really about 61.8 mph, and the axle sits ~0.37″ higher.
Reading this comparison
The overall diameter is what matters here: 245/40R18 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.