37x12.50R18 vs 325/60R20

325/60R20 is 1.65 in (41.8 mm) shorter than 37x12.50R18, a −4.45% diameter difference. With the speedometer set for 37x12.50R18, an indicated 60 mph is 57.3 mph on 325/60R20.

Is this a safe swap?

Diameter difference −4.45%: outside the ±3% guideline

By overall diameter alone, 325/60R20 falls outside the ±3% range commonly used as a guideline when replacing 37x12.50R18; 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.

37x12.50R18 vs 325/60R20 differences
Measurement	37x12.50R18	325/60R20	Difference	% diff
Overall diameter	37.00 in (939.8 mm)	35.35 in (898.0 mm)	−1.65 in (−41.8 mm)	−4.45%
Sidewall height	9.50 in (241.3 mm)	7.68 in (195.0 mm)	−1.82 in (−46.3 mm)	−19.19%
Section width	12.50 in (317.5 mm)	12.80 in (325.0 mm)	+0.30 in (+7.5 mm)	+2.36%
Circumference	116.24 in (2,952.5 mm)	111.07 in (2,821.2 mm)	−5.17 in (−131.3 mm)	−4.45%
Revolutions per mile	545	570	+25	+4.65%

True speed with the speedometer calibrated for 37x12.50R18
Indicated	Actual on 37x12.50R18	Actual on 325/60R20	Difference
30 mph	30.0 mph	28.7 mph	−1.3 mph
60 mph	60.0 mph	57.3 mph	−2.7 mph
75 mph	75.0 mph	71.7 mph	−3.3 mph

Ride-height change: 325/60R20 lowers the vehicle by 0.82 in (20.9 mm) at the axle (half the −41.8 mm diameter difference).

325/60R20 is 1.65 in shorter than 37x12.50R18 and 8 mm wider, a 4.45% difference in overall diameter.

With the speedometer calibrated for 37x12.50R18, an indicated 60 mph is actually 57.3 mph on 325/60R20, and revolutions per mile change from 545 to 570.

325/60R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 37x12.50R18.

Size pages

Other comparisons with 37x12.50R18

Other comparisons with 325/60R20

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