285/65R18 vs 33x12.50R20

33x12.50R20 is 0.41 in (10.5 mm) taller than 285/65R18, a +1.27% diameter difference. With the speedometer set for 285/65R18, an indicated 60 mph is 60.8 mph on 33x12.50R20.

Is this a safe swap?

Diameter difference +1.27%: within the ±3% guideline

By overall diameter alone, 33x12.50R20 falls inside the ±3% range commonly used as a guideline when replacing 285/65R18. 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.

285/65R18 vs 33x12.50R20 differences
Measurement	285/65R18	33x12.50R20	Difference	% diff
Overall diameter	32.59 in (827.7 mm)	33.00 in (838.2 mm)	+0.41 in (+10.5 mm)	+1.27%
Sidewall height	7.29 in (185.3 mm)	6.50 in (165.1 mm)	−0.79 in (−20.2 mm)	−10.88%
Section width	11.22 in (285.0 mm)	12.50 in (317.5 mm)	+1.28 in (+32.5 mm)	+11.40%
Circumference	102.37 in (2,600.3 mm)	103.67 in (2,633.3 mm)	+1.30 in (+33.0 mm)	+1.27%
Revolutions per mile	619	611	−8	−1.25%

True speed with the speedometer calibrated for 285/65R18
Indicated	Actual on 285/65R18	Actual on 33x12.50R20	Difference
30 mph	30.0 mph	30.4 mph	+0.4 mph
60 mph	60.0 mph	60.8 mph	+0.8 mph
75 mph	75.0 mph	76.0 mph	+1.0 mph

Ride-height change: 33x12.50R20 raises the vehicle by 0.21 in (5.3 mm) at the axle (half the +10.5 mm diameter difference).

33x12.50R20 is 0.41 in taller than 285/65R18 and 33 mm wider, a 1.27% difference in overall diameter.

With the speedometer calibrated for 285/65R18, an indicated 60 mph is actually 60.8 mph on 33x12.50R20, and revolutions per mile change from 619 to 611.

33x12.50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/65R18.

Size pages

Other comparisons with 285/65R18

Other comparisons with 33x12.50R20

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