TireSizesFinder

37x13.50R18 vs 38x13.50R20

38x13.50R20 is 1.00 in (25.4 mm) taller than 37x13.50R18, a +2.70% diameter difference. With the speedometer set for 37x13.50R18, an indicated 60 mph is 61.6 mph on 38x13.50R20.

Is this a safe swap?

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

By overall diameter alone, 38x13.50R20 falls inside the ±3% range commonly used as a guideline when replacing 37x13.50R18. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 37x13.50R18 (37.00 in) vs 38x13.50R20 (38.00 in)37x13.50R1837.00 in × 13.50 in38x13.50R2038.00 in × 13.50 in
Side profile and tread width, drawn to the same scale.
37x13.50R18 vs 38x13.50R20 differences
Measurement37x13.50R1838x13.50R20Difference% diff
Overall diameter37.00 in (939.8 mm)38.00 in (965.2 mm)+1.00 in (+25.4 mm)+2.70%
Sidewall height9.50 in (241.3 mm)9.00 in (228.6 mm)−0.50 in (−12.7 mm)−5.26%
Section width13.50 in (342.9 mm)13.50 in (342.9 mm)0.00 in (0.0 mm)0.00%
Circumference116.24 in (2,952.5 mm)119.38 in (3,032.3 mm)+3.14 in (+79.8 mm)+2.70%
Revolutions per mile545531−14−2.63%
True speed with the speedometer calibrated for 37x13.50R18
IndicatedActual on 37x13.50R18Actual on 38x13.50R20Difference
30 mph30.0 mph30.8 mph+0.8 mph
60 mph60.0 mph61.6 mph+1.6 mph
75 mph75.0 mph77.0 mph+2.0 mph

Ride-height change: 38x13.50R20 raises the vehicle by 0.50 in (12.7 mm) at the axle (half the +25.4 mm diameter difference).

38x13.50R20 is 1.00 in taller than 37x13.50R18 and the same width, a 2.70% difference in overall diameter.

With the speedometer calibrated for 37x13.50R18, an indicated 60 mph is actually 61.6 mph on 38x13.50R20, and revolutions per mile change from 545 to 531.

38x13.50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 37x13.50R18.

Size pages

Other comparisons with 37x13.50R18

Other comparisons with 38x13.50R20

Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.