275/65R18 vs 285/55R20

285/55R20 is 0.27 in (6.8 mm) taller than 275/65R18, a +0.83% diameter difference. With the speedometer set for 275/65R18, an indicated 60 mph is 60.5 mph on 285/55R20.

Is this a safe swap?

Diameter difference +0.83%: under 1%

By overall diameter alone, 285/55R20 is a near match for 275/65R18; 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.

275/65R18 vs 285/55R20 differences
Measurement	275/65R18	285/55R20	Difference	% diff
Overall diameter	32.07 in (814.7 mm)	32.34 in (821.5 mm)	+0.27 in (+6.8 mm)	+0.83%
Sidewall height	7.04 in (178.8 mm)	6.17 in (156.8 mm)	−0.87 in (−22.0 mm)	−12.31%
Section width	10.83 in (275.0 mm)	11.22 in (285.0 mm)	+0.39 in (+10.0 mm)	+3.64%
Circumference	100.77 in (2,559.5 mm)	101.61 in (2,580.8 mm)	+0.84 in (+21.4 mm)	+0.83%
Revolutions per mile	629	624	−5	−0.83%

True speed with the speedometer calibrated for 275/65R18
Indicated	Actual on 275/65R18	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	75.6 mph	+0.6 mph

Ride-height change: 285/55R20 raises the vehicle by 0.13 in (3.4 mm) at the axle (half the +6.8 mm diameter difference).

285/55R20 is 0.27 in taller than 275/65R18 and 10 mm wider, a 0.83% difference in overall diameter.

With the speedometer calibrated for 275/65R18, 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 275/65R18.

Size pages

Other comparisons with 275/65R18

Other comparisons with 285/55R20

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