285/75R18 vs 315/60R20

315/60R20 is 0.05 in (1.3 mm) taller than 285/75R18, a +0.15% diameter difference. With the speedometer set for 285/75R18, an indicated 60 mph is 60.1 mph on 315/60R20.

Is this a safe swap?

Diameter difference +0.15%: under 1%

By overall diameter alone, 315/60R20 is a near match for 285/75R18; 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.

285/75R18 vs 315/60R20 differences
Measurement	285/75R18	315/60R20	Difference	% diff
Overall diameter	34.83 in (884.7 mm)	34.88 in (886.0 mm)	+0.05 in (+1.3 mm)	+0.15%
Sidewall height	8.42 in (213.8 mm)	7.44 in (189.0 mm)	−0.97 in (−24.7 mm)	−11.58%
Section width	11.22 in (285.0 mm)	12.40 in (315.0 mm)	+1.18 in (+30.0 mm)	+10.53%
Circumference	109.42 in (2,779.4 mm)	109.58 in (2,783.5 mm)	+0.16 in (+4.1 mm)	+0.15%
Revolutions per mile	579	578	−1	−0.15%

True speed with the speedometer calibrated for 285/75R18
Indicated	Actual on 285/75R18	Actual on 315/60R20	Difference
30 mph	30.0 mph	30.0 mph	0.0 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: 315/60R20 raises the vehicle by 0.03 in (0.6 mm) at the axle (half the +1.3 mm diameter difference).

315/60R20 is 0.05 in taller than 285/75R18 and 30 mm wider, a 0.15% difference in overall diameter.

With the speedometer calibrated for 285/75R18, an indicated 60 mph is actually 60.1 mph on 315/60R20, and revolutions per mile change from 579 to 578.

315/60R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 285/75R18.

Size pages

Other comparisons with 285/75R18

Other comparisons with 315/60R20

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