ElpiAuto235/45R18 vs 255/40R19
255/40R19 is 0.70″ (18 mm) taller than 235/45R18, a 2.7% 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 | 235/45R18 | 255/40R19 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 26.33″ | 27.03″ | +0.70″ | +2.7% |
| Section width | 9.25″ | 10.04″ | +0.79″ | +8.5% |
| Sidewall height | 4.16″ | 4.02″ | −0.15″ | −3.5% |
| Circumference | 82.71″ | 84.92″ | +2.21″ | +2.7% |
| Revs per mile | 766.1 | 746.1 | −20.0 | −2.6% |
| Revs per km | 476.0 | 463.6 | −12.4 | −2.6% |
Speedometer: with 235/45R18 as the original, 255/40R19 makes an indicated 60 mph really about 61.6 mph, and the axle sits ~0.35″ higher.
Reading this comparison
The overall diameter is what matters here: 255/40R19 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.