305/70R18 vs 315/60R20

315/60R20 is 0.07 in (1.8 mm) taller than 305/70R18, a +0.20% diameter difference. With the speedometer set for 305/70R18, an indicated 60 mph is 60.1 mph on 315/60R20.

Is this a safe swap?

Diameter difference +0.20%: under 1%

By overall diameter alone, 315/60R20 is a near match for 305/70R18; 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.

305/70R18 vs 315/60R20 differences
Measurement	305/70R18	315/60R20	Difference	% diff
Overall diameter	34.81 in (884.2 mm)	34.88 in (886.0 mm)	+0.07 in (+1.8 mm)	+0.20%
Sidewall height	8.41 in (213.5 mm)	7.44 in (189.0 mm)	−0.96 in (−24.5 mm)	−11.48%
Section width	12.01 in (305.0 mm)	12.40 in (315.0 mm)	+0.39 in (+10.0 mm)	+3.28%
Circumference	109.36 in (2,777.8 mm)	109.58 in (2,783.5 mm)	+0.22 in (+5.7 mm)	+0.20%
Revolutions per mile	579	578	−1	−0.20%

True speed with the speedometer calibrated for 305/70R18
Indicated	Actual on 305/70R18	Actual on 315/60R20	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	75.2 mph	+0.2 mph

Ride-height change: 315/60R20 raises the vehicle by 0.04 in (0.9 mm) at the axle (half the +1.8 mm diameter difference).

315/60R20 is 0.07 in taller than 305/70R18 and 10 mm wider, a 0.20% difference in overall diameter.

With the speedometer calibrated for 305/70R18, 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 305/70R18.

Size pages

Other comparisons with 305/70R18

Other comparisons with 315/60R20

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