275/70R18 vs 33x12.50R20

33x12.50R20 is 0.16 in (4.0 mm) shorter than 275/70R18, a −0.47% diameter difference. With the speedometer set for 275/70R18, an indicated 60 mph is 59.7 mph on 33x12.50R20.

Is this a safe swap?

Diameter difference −0.47%: under 1%

By overall diameter alone, 33x12.50R20 is a near match for 275/70R18; the speedometer changes by less than 1%. 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.

275/70R18 vs 33x12.50R20 differences
Measurement	275/70R18	33x12.50R20	Difference	% diff
Overall diameter	33.16 in (842.2 mm)	33.00 in (838.2 mm)	−0.16 in (−4.0 mm)	−0.47%
Sidewall height	7.58 in (192.5 mm)	6.50 in (165.1 mm)	−1.08 in (−27.4 mm)	−14.23%
Section width	10.83 in (275.0 mm)	12.50 in (317.5 mm)	+1.67 in (+42.5 mm)	+15.45%
Circumference	104.17 in (2,645.8 mm)	103.67 in (2,633.3 mm)	−0.49 in (−12.6 mm)	−0.47%
Revolutions per mile	608	611	+3	+0.48%

True speed with the speedometer calibrated for 275/70R18
Indicated	Actual on 275/70R18	Actual on 33x12.50R20	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.6 mph	−0.4 mph

Ride-height change: 33x12.50R20 lowers the vehicle by 0.08 in (2.0 mm) at the axle (half the −4.0 mm diameter difference).

33x12.50R20 is 0.16 in shorter than 275/70R18 and 43 mm wider, a 0.47% difference in overall diameter.

With the speedometer calibrated for 275/70R18, an indicated 60 mph is actually 59.7 mph on 33x12.50R20, and revolutions per mile change from 608 to 611.

33x12.50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 275/70R18.

Size pages

Other comparisons with 275/70R18

Other comparisons with 33x12.50R20

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