285/35R20 vs 255/35R21

255/35R21 is 0.17 in (4.4 mm) taller than 285/35R20, a +0.62% diameter difference. With the speedometer set for 285/35R20, an indicated 60 mph is 60.4 mph on 255/35R21.

Is this a safe swap?

Diameter difference +0.62%: under 1%

By overall diameter alone, 255/35R21 is a near match for 285/35R20; 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.

285/35R20 vs 255/35R21 differences
Measurement	285/35R20	255/35R21	Difference	% diff
Overall diameter	27.85 in (707.5 mm)	28.03 in (711.9 mm)	+0.17 in (+4.4 mm)	+0.62%
Sidewall height	3.93 in (99.8 mm)	3.51 in (89.3 mm)	−0.41 in (−10.5 mm)	−10.53%
Section width	11.22 in (285.0 mm)	10.04 in (255.0 mm)	−1.18 in (−30.0 mm)	−10.53%
Circumference	87.51 in (2,222.7 mm)	88.05 in (2,236.5 mm)	+0.54 in (+13.8 mm)	+0.62%
Revolutions per mile	724	720	−4	−0.62%

True speed with the speedometer calibrated for 285/35R20
Indicated	Actual on 285/35R20	Actual on 255/35R21	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 255/35R21 raises the vehicle by 0.09 in (2.2 mm) at the axle (half the +4.4 mm diameter difference).

255/35R21 is 0.17 in taller than 285/35R20 and 30 mm narrower, a 0.62% difference in overall diameter.

With the speedometer calibrated for 285/35R20, an indicated 60 mph is actually 60.4 mph on 255/35R21, and revolutions per mile change from 724 to 720.

255/35R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 285/35R20.

Size pages

Other comparisons with 285/35R20

Other comparisons with 255/35R21

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