255/65R18 vs 285/50R20

285/50R20 is 0.17 in (4.3 mm) taller than 255/65R18, a +0.55% diameter difference. With the speedometer set for 255/65R18, an indicated 60 mph is 60.3 mph on 285/50R20.

Is this a safe swap?

Diameter difference +0.55%: under 1%

By overall diameter alone, 285/50R20 is a near match for 255/65R18; 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.

255/65R18 vs 285/50R20 differences
Measurement	255/65R18	285/50R20	Difference	% diff
Overall diameter	31.05 in (788.7 mm)	31.22 in (793.0 mm)	+0.17 in (+4.3 mm)	+0.55%
Sidewall height	6.53 in (165.8 mm)	5.61 in (142.5 mm)	−0.92 in (−23.2 mm)	−14.03%
Section width	10.04 in (255.0 mm)	11.22 in (285.0 mm)	+1.18 in (+30.0 mm)	+11.76%
Circumference	97.55 in (2,477.8 mm)	98.08 in (2,491.3 mm)	+0.53 in (+13.5 mm)	+0.55%
Revolutions per mile	650	646	−4	−0.54%

True speed with the speedometer calibrated for 255/65R18
Indicated	Actual on 255/65R18	Actual on 285/50R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 285/50R20 raises the vehicle by 0.08 in (2.1 mm) at the axle (half the +4.3 mm diameter difference).

285/50R20 is 0.17 in taller than 255/65R18 and 30 mm wider, a 0.55% difference in overall diameter.

With the speedometer calibrated for 255/65R18, an indicated 60 mph is actually 60.3 mph on 285/50R20, and revolutions per mile change from 650 to 646.

285/50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/65R18.

Size pages

Other comparisons with 255/65R18

Other comparisons with 285/50R20

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