255/50R18 vs 285/35R20

285/35R20 is 0.19 in (4.7 mm) shorter than 255/50R18, a −0.66% diameter difference. With the speedometer set for 255/50R18, an indicated 60 mph is 59.6 mph on 285/35R20.

Is this a safe swap?

Diameter difference −0.66%: under 1%

By overall diameter alone, 285/35R20 is a near match for 255/50R18; 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.

255/50R18 vs 285/35R20 differences
Measurement	255/50R18	285/35R20	Difference	% diff
Overall diameter	28.04 in (712.2 mm)	27.85 in (707.5 mm)	−0.19 in (−4.7 mm)	−0.66%
Sidewall height	5.02 in (127.5 mm)	3.93 in (99.8 mm)	−1.09 in (−27.7 mm)	−21.76%
Section width	10.04 in (255.0 mm)	11.22 in (285.0 mm)	+1.18 in (+30.0 mm)	+11.76%
Circumference	88.09 in (2,237.4 mm)	87.51 in (2,222.7 mm)	−0.58 in (−14.8 mm)	−0.66%
Revolutions per mile	719	724	+5	+0.66%

True speed with the speedometer calibrated for 255/50R18
Indicated	Actual on 255/50R18	Actual on 285/35R20	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.6 mph	−0.4 mph
75 mph	75.0 mph	74.5 mph	−0.5 mph

Ride-height change: 285/35R20 lowers the vehicle by 0.09 in (2.4 mm) at the axle (half the −4.7 mm diameter difference).

285/35R20 is 0.19 in shorter than 255/50R18 and 30 mm wider, a 0.66% difference in overall diameter.

With the speedometer calibrated for 255/50R18, an indicated 60 mph is actually 59.6 mph on 285/35R20, and revolutions per mile change from 719 to 724.

285/35R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/50R18.

Size pages

Other comparisons with 255/50R18

Other comparisons with 285/35R20

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