315/70R18 vs 37x12.50R20

37x12.50R20 is 1.64 in (41.6 mm) taller than 315/70R18, a +4.63% diameter difference. With the speedometer set for 315/70R18, an indicated 60 mph is 62.8 mph on 37x12.50R20.

Is this a safe swap?

Diameter difference +4.63%: outside the ±3% guideline

By overall diameter alone, 37x12.50R20 falls outside the ±3% range commonly used as a guideline when replacing 315/70R18; speedometer and ride-height changes are larger. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

315/70R18 vs 37x12.50R20 differences
Measurement	315/70R18	37x12.50R20	Difference	% diff
Overall diameter	35.36 in (898.2 mm)	37.00 in (939.8 mm)	+1.64 in (+41.6 mm)	+4.63%
Sidewall height	8.68 in (220.5 mm)	8.50 in (215.9 mm)	−0.18 in (−4.6 mm)	−2.09%
Section width	12.40 in (315.0 mm)	12.50 in (317.5 mm)	+0.10 in (+2.5 mm)	+0.79%
Circumference	111.09 in (2,821.8 mm)	116.24 in (2,952.5 mm)	+5.15 in (+130.7 mm)	+4.63%
Revolutions per mile	570	545	−25	−4.43%

True speed with the speedometer calibrated for 315/70R18
Indicated	Actual on 315/70R18	Actual on 37x12.50R20	Difference
30 mph	30.0 mph	31.4 mph	+1.4 mph
60 mph	60.0 mph	62.8 mph	+2.8 mph
75 mph	75.0 mph	78.5 mph	+3.5 mph

Ride-height change: 37x12.50R20 raises the vehicle by 0.82 in (20.8 mm) at the axle (half the +41.6 mm diameter difference).

37x12.50R20 is 1.64 in taller than 315/70R18 and 3 mm wider, a 4.63% difference in overall diameter.

With the speedometer calibrated for 315/70R18, an indicated 60 mph is actually 62.8 mph on 37x12.50R20, and revolutions per mile change from 570 to 545.

37x12.50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 315/70R18.

Size pages

Other comparisons with 315/70R18

Other comparisons with 37x12.50R20

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.