285/70R18 vs 33x12.50R20

33x12.50R20 is 0.71 in (18.0 mm) shorter than 285/70R18, a −2.10% diameter difference. With the speedometer set for 285/70R18, an indicated 60 mph is 58.7 mph on 33x12.50R20.

Is this a safe swap?

Diameter difference −2.10%: within the ±3% guideline

By overall diameter alone, 33x12.50R20 falls inside the ±3% range commonly used as a guideline when replacing 285/70R18. 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.

285/70R18 vs 33x12.50R20 differences
Measurement	285/70R18	33x12.50R20	Difference	% diff
Overall diameter	33.71 in (856.2 mm)	33.00 in (838.2 mm)	−0.71 in (−18.0 mm)	−2.10%
Sidewall height	7.85 in (199.5 mm)	6.50 in (165.1 mm)	−1.35 in (−34.4 mm)	−17.24%
Section width	11.22 in (285.0 mm)	12.50 in (317.5 mm)	+1.28 in (+32.5 mm)	+11.40%
Circumference	105.90 in (2,689.8 mm)	103.67 in (2,633.3 mm)	−2.23 in (−56.5 mm)	−2.10%
Revolutions per mile	598	611	+13	+2.15%

True speed with the speedometer calibrated for 285/70R18
Indicated	Actual on 285/70R18	Actual on 33x12.50R20	Difference
30 mph	30.0 mph	29.4 mph	−0.6 mph
60 mph	60.0 mph	58.7 mph	−1.3 mph
75 mph	75.0 mph	73.4 mph	−1.6 mph

Ride-height change: 33x12.50R20 lowers the vehicle by 0.35 in (9.0 mm) at the axle (half the −18.0 mm diameter difference).

33x12.50R20 is 0.71 in shorter than 285/70R18 and 33 mm wider, a 2.10% difference in overall diameter.

With the speedometer calibrated for 285/70R18, an indicated 60 mph is actually 58.7 mph on 33x12.50R20, and revolutions per mile change from 598 to 611.

33x12.50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/70R18.

Size pages

Other comparisons with 285/70R18

Other comparisons with 33x12.50R20

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