255/70R18 vs 285/55R20

285/55R20 is 0.29 in (7.3 mm) taller than 255/70R18, a +0.90% diameter difference. With the speedometer set for 255/70R18, an indicated 60 mph is 60.5 mph on 285/55R20.

Is this a safe swap?

Diameter difference +0.90%: under 1%

By overall diameter alone, 285/55R20 is a near match for 255/70R18; 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.

255/70R18 vs 285/55R20 differences
Measurement	255/70R18	285/55R20	Difference	% diff
Overall diameter	32.06 in (814.2 mm)	32.34 in (821.5 mm)	+0.29 in (+7.3 mm)	+0.90%
Sidewall height	7.03 in (178.5 mm)	6.17 in (156.8 mm)	−0.86 in (−21.7 mm)	−12.18%
Section width	10.04 in (255.0 mm)	11.22 in (285.0 mm)	+1.18 in (+30.0 mm)	+11.76%
Circumference	100.70 in (2,557.9 mm)	101.61 in (2,580.8 mm)	+0.90 in (+22.9 mm)	+0.90%
Revolutions per mile	629	624	−6	−0.89%

True speed with the speedometer calibrated for 255/70R18
Indicated	Actual on 255/70R18	Actual on 285/55R20	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 285/55R20 raises the vehicle by 0.14 in (3.6 mm) at the axle (half the +7.3 mm diameter difference).

285/55R20 is 0.29 in taller than 255/70R18 and 30 mm wider, a 0.90% difference in overall diameter.

With the speedometer calibrated for 255/70R18, an indicated 60 mph is actually 60.5 mph on 285/55R20, and revolutions per mile change from 629 to 624.

285/55R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/70R18.

Size pages

Other comparisons with 255/70R18

Other comparisons with 285/55R20

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