ElpiAuto195/65R15 vs 245/40R18
245/40R18 is 0.74″ (19 mm) taller than 195/65R15, a 2.9% 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.
The section widths are very different — these are not a like-for-like swap. Confirm wheel width and arch clearance separately.
| Measurement | 195/65R15 | 245/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 24.98″ | 25.72″ | +0.74″ | +2.9% |
| Section width | 7.68″ | 9.65″ | +1.97″ | +25.6% |
| Sidewall height | 4.99″ | 3.86″ | −1.13″ | −22.7% |
| Circumference | 78.48″ | 80.79″ | +2.31″ | +2.9% |
| Revs per mile | 807.4 | 784.2 | −23.1 | −2.9% |
| Revs per km | 501.7 | 487.3 | −14.4 | −2.9% |
Speedometer: with 195/65R15 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.