255/55R20 vs 285/45R21

285/45R21 is 0.06 in (1.4 mm) taller than 255/55R20, a +0.18% diameter difference. With the speedometer set for 255/55R20, an indicated 60 mph is 60.1 mph on 285/45R21.

Is this a safe swap?

Diameter difference +0.18%: under 1%

By overall diameter alone, 285/45R21 is a near match for 255/55R20; 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.

255/55R20 vs 285/45R21 differences
Measurement	255/55R20	285/45R21	Difference	% diff
Overall diameter	31.04 in (788.5 mm)	31.10 in (789.9 mm)	+0.06 in (+1.4 mm)	+0.18%
Sidewall height	5.52 in (140.3 mm)	5.05 in (128.3 mm)	−0.47 in (−12.0 mm)	−8.56%
Section width	10.04 in (255.0 mm)	11.22 in (285.0 mm)	+1.18 in (+30.0 mm)	+11.76%
Circumference	97.53 in (2,477.1 mm)	97.70 in (2,481.5 mm)	+0.17 in (+4.4 mm)	+0.18%
Revolutions per mile	650	649	−1	−0.18%

True speed with the speedometer calibrated for 255/55R20
Indicated	Actual on 255/55R20	Actual on 285/45R21	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 285/45R21 raises the vehicle by 0.03 in (0.7 mm) at the axle (half the +1.4 mm diameter difference).

285/45R21 is 0.06 in taller than 255/55R20 and 30 mm wider, a 0.18% difference in overall diameter.

With the speedometer calibrated for 255/55R20, an indicated 60 mph is actually 60.1 mph on 285/45R21, and revolutions per mile change from 650 to 649.

285/45R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/55R20.

Size pages

Other comparisons with 255/55R20

Other comparisons with 285/45R21

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