255/85R16 vs 33x12.50R18

33x12.50R18 is 0.07 in (1.7 mm) shorter than 255/85R16, a −0.20% diameter difference. With the speedometer set for 255/85R16, an indicated 60 mph is 59.9 mph on 33x12.50R18.

Is this a safe swap?

Diameter difference −0.20%: under 1%

By overall diameter alone, 33x12.50R18 is a near match for 255/85R16; 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.

255/85R16 vs 33x12.50R18 differences
Measurement	255/85R16	33x12.50R18	Difference	% diff
Overall diameter	33.07 in (839.9 mm)	33.00 in (838.2 mm)	−0.07 in (−1.7 mm)	−0.20%
Sidewall height	8.53 in (216.8 mm)	7.50 in (190.5 mm)	−1.03 in (−26.2 mm)	−12.11%
Section width	10.04 in (255.0 mm)	12.50 in (317.5 mm)	+2.46 in (+62.5 mm)	+24.51%
Circumference	103.88 in (2,638.6 mm)	103.67 in (2,633.3 mm)	−0.21 in (−5.3 mm)	−0.20%
Revolutions per mile	610	611	+1	+0.20%

True speed with the speedometer calibrated for 255/85R16
Indicated	Actual on 255/85R16	Actual on 33x12.50R18	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.9 mph	−0.1 mph
75 mph	75.0 mph	74.8 mph	−0.2 mph

Ride-height change: 33x12.50R18 lowers the vehicle by 0.03 in (0.8 mm) at the axle (half the −1.7 mm diameter difference).

33x12.50R18 is 0.07 in shorter than 255/85R16 and 63 mm wider, a 0.20% difference in overall diameter.

With the speedometer calibrated for 255/85R16, an indicated 60 mph is actually 59.9 mph on 33x12.50R18, and revolutions per mile change from 610 to 611.

33x12.50R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 255/85R16.

Size pages

Other comparisons with 255/85R16

Other comparisons with 33x12.50R18

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