285/65R18 vs 285/55R20

285/55R20 is 0.24 in (6.2 mm) shorter than 285/65R18, a −0.75% diameter difference. With the speedometer set for 285/65R18, an indicated 60 mph is 59.6 mph on 285/55R20.

Is this a safe swap?

Diameter difference −0.75%: under 1%

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

285/65R18 vs 285/55R20 differences
Measurement	285/65R18	285/55R20	Difference	% diff
Overall diameter	32.59 in (827.7 mm)	32.34 in (821.5 mm)	−0.24 in (−6.2 mm)	−0.75%
Sidewall height	7.29 in (185.3 mm)	6.17 in (156.8 mm)	−1.12 in (−28.5 mm)	−15.38%
Section width	11.22 in (285.0 mm)	11.22 in (285.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	102.37 in (2,600.3 mm)	101.61 in (2,580.8 mm)	−0.77 in (−19.5 mm)	−0.75%
Revolutions per mile	619	624	+5	+0.75%

True speed with the speedometer calibrated for 285/65R18
Indicated	Actual on 285/65R18	Actual on 285/55R20	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	74.4 mph	−0.6 mph

Ride-height change: 285/55R20 lowers the vehicle by 0.12 in (3.1 mm) at the axle (half the −6.2 mm diameter difference).

285/55R20 is 0.24 in shorter than 285/65R18 and the same width, a 0.75% difference in overall diameter.

With the speedometer calibrated for 285/65R18, an indicated 60 mph is actually 59.6 mph on 285/55R20, and revolutions per mile change from 619 to 624.

285/55R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/65R18.

Size pages

Other comparisons with 285/65R18

Other comparisons with 285/55R20

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