ElpiAuto205/55R16 vs 245/40R18
245/40R18 is 0.84″ (21 mm) taller than 205/55R16, a 3.4% difference in overall diameter — outside the ±3% band, so expect the speedometer to drift and check for rubbing before fitting.
These sit on different wheel sizes — you'd be changing wheels too, not just tires.
| Measurement | 205/55R16 | 245/40R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 24.88″ | 25.72″ | +0.84″ | +3.4% |
| Section width | 8.07″ | 9.65″ | +1.57″ | +19.5% |
| Sidewall height | 4.44″ | 3.86″ | −0.58″ | −13.1% |
| Circumference | 78.16″ | 80.79″ | +2.63″ | +3.4% |
| Revs per mile | 810.7 | 784.2 | −26.4 | −3.3% |
| Revs per km | 503.7 | 487.3 | −16.4 | −3.3% |
Speedometer: with 205/55R16 as the original, 245/40R18 makes an indicated 60 mph really about 62.0 mph, and the axle sits ~0.42″ 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.