285/40R20 vs 285/35R21

285/35R21 is 0.12 in (3.1 mm) shorter than 285/40R20, a −0.42% diameter difference. With the speedometer set for 285/40R20, an indicated 60 mph is 59.7 mph on 285/35R21.

Is this a safe swap?

Diameter difference −0.42%: under 1%

By overall diameter alone, 285/35R21 is a near match for 285/40R20; 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.

285/40R20 vs 285/35R21 differences
Measurement	285/40R20	285/35R21	Difference	% diff
Overall diameter	28.98 in (736.0 mm)	28.85 in (732.9 mm)	−0.12 in (−3.1 mm)	−0.42%
Sidewall height	4.49 in (114.0 mm)	3.93 in (99.8 mm)	−0.56 in (−14.2 mm)	−12.50%
Section width	11.22 in (285.0 mm)	11.22 in (285.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	91.03 in (2,312.2 mm)	90.65 in (2,302.5 mm)	−0.38 in (−9.7 mm)	−0.42%
Revolutions per mile	696	699	+3	+0.42%

True speed with the speedometer calibrated for 285/40R20
Indicated	Actual on 285/40R20	Actual on 285/35R21	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.7 mph	−0.3 mph

Ride-height change: 285/35R21 lowers the vehicle by 0.06 in (1.6 mm) at the axle (half the −3.1 mm diameter difference).

285/35R21 is 0.12 in shorter than 285/40R20 and the same width, a 0.42% difference in overall diameter.

With the speedometer calibrated for 285/40R20, an indicated 60 mph is actually 59.7 mph on 285/35R21, and revolutions per mile change from 696 to 699.

285/35R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 285/40R20.

Size pages

Other comparisons with 285/40R20

Other comparisons with 285/35R21

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