315/35R20 vs 285/35R21

285/35R21 is 0.17 in (4.4 mm) taller than 315/35R20, a +0.60% diameter difference. With the speedometer set for 315/35R20, an indicated 60 mph is 60.4 mph on 285/35R21.

Is this a safe swap?

Diameter difference +0.60%: under 1%

By overall diameter alone, 285/35R21 is a near match for 315/35R20; 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.

315/35R20 vs 285/35R21 differences
Measurement	315/35R20	285/35R21	Difference	% diff
Overall diameter	28.68 in (728.5 mm)	28.85 in (732.9 mm)	+0.17 in (+4.4 mm)	+0.60%
Sidewall height	4.34 in (110.3 mm)	3.93 in (99.8 mm)	−0.41 in (−10.5 mm)	−9.52%
Section width	12.40 in (315.0 mm)	11.22 in (285.0 mm)	−1.18 in (−30.0 mm)	−9.52%
Circumference	90.10 in (2,288.7 mm)	90.65 in (2,302.5 mm)	+0.54 in (+13.8 mm)	+0.60%
Revolutions per mile	703	699	−4	−0.60%

True speed with the speedometer calibrated for 315/35R20
Indicated	Actual on 315/35R20	Actual on 285/35R21	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: 285/35R21 raises the vehicle by 0.09 in (2.2 mm) at the axle (half the +4.4 mm diameter difference).

285/35R21 is 0.17 in taller than 315/35R20 and 30 mm narrower, a 0.60% difference in overall diameter.

With the speedometer calibrated for 315/35R20, an indicated 60 mph is actually 60.4 mph on 285/35R21, and revolutions per mile change from 703 to 699.

285/35R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 315/35R20.

Size pages

Other comparisons with 315/35R20

Other comparisons with 285/35R21

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