ElpiAuto305/70R18 vs 37X12.50R18
37X12.50R18 is 2.19″ (56 mm) taller than 305/70R18, a 6.3% difference in overall diameter — outside the ±3% band, so expect the speedometer to drift and check for rubbing before fitting.
| Measurement | 305/70R18 | 37X12.50R18 | Difference | % |
|---|---|---|---|---|
| Overall diameter | 34.81″ | 37.00″ | +2.19″ | +6.3% |
| Section width | 12.01″ | 12.50″ | +0.49″ | +4.1% |
| Sidewall height | 8.41″ | 9.50″ | +1.09″ | +13.0% |
| Circumference | 109.36″ | 116.24″ | +6.88″ | +6.3% |
| Revs per mile | 579.4 | 545.1 | −34.3 | −5.9% |
| Revs per km | 360.0 | 338.7 | −21.3 | −5.9% |
Speedometer: with 305/70R18 as the original, 37X12.50R18 makes an indicated 60 mph really about 63.8 mph, and the axle sits ~1.09″ higher.
Reading this comparison
The overall diameter is what matters here: 37X12.50R18 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.