37x12.50R20 vs 38x15.50R20

38x15.50R20 is 1.00 in (25.4 mm) taller than 37x12.50R20, a +2.70% diameter difference. With the speedometer set for 37x12.50R20, an indicated 60 mph is 61.6 mph on 38x15.50R20.

Is this a safe swap?

Diameter difference +2.70%: within the ±3% guideline

By overall diameter alone, 38x15.50R20 falls inside the ±3% range commonly used as a guideline when replacing 37x12.50R20. 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.

37x12.50R20 vs 38x15.50R20 differences
Measurement	37x12.50R20	38x15.50R20	Difference	% diff
Overall diameter	37.00 in (939.8 mm)	38.00 in (965.2 mm)	+1.00 in (+25.4 mm)	+2.70%
Sidewall height	8.50 in (215.9 mm)	9.00 in (228.6 mm)	+0.50 in (+12.7 mm)	+5.88%
Section width	12.50 in (317.5 mm)	15.50 in (393.7 mm)	+3.00 in (+76.2 mm)	+24.00%
Circumference	116.24 in (2,952.5 mm)	119.38 in (3,032.3 mm)	+3.14 in (+79.8 mm)	+2.70%
Revolutions per mile	545	531	−14	−2.63%

True speed with the speedometer calibrated for 37x12.50R20
Indicated	Actual on 37x12.50R20	Actual on 38x15.50R20	Difference
30 mph	30.0 mph	30.8 mph	+0.8 mph
60 mph	60.0 mph	61.6 mph	+1.6 mph
75 mph	75.0 mph	77.0 mph	+2.0 mph

Ride-height change: 38x15.50R20 raises the vehicle by 0.50 in (12.7 mm) at the axle (half the +25.4 mm diameter difference).

38x15.50R20 is 1.00 in taller than 37x12.50R20 and 76 mm wider, a 2.70% difference in overall diameter.

With the speedometer calibrated for 37x12.50R20, an indicated 60 mph is actually 61.6 mph on 38x15.50R20, and revolutions per mile change from 545 to 531.

Both sizes mount on 20-inch wheels.

Size pages

Other comparisons with 37x12.50R20

Other comparisons with 38x15.50R20

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