315/70R18 vs 315/60R20

315/60R20 is 0.48 in (12.2 mm) shorter than 315/70R18, a −1.36% diameter difference. With the speedometer set for 315/70R18, an indicated 60 mph is 59.2 mph on 315/60R20.

Is this a safe swap?

Diameter difference −1.36%: within the ±3% guideline

By overall diameter alone, 315/60R20 falls inside the ±3% range commonly used as a guideline when replacing 315/70R18. 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.

315/70R18 vs 315/60R20 differences
Measurement	315/70R18	315/60R20	Difference	% diff
Overall diameter	35.36 in (898.2 mm)	34.88 in (886.0 mm)	−0.48 in (−12.2 mm)	−1.36%
Sidewall height	8.68 in (220.5 mm)	7.44 in (189.0 mm)	−1.24 in (−31.5 mm)	−14.29%
Section width	12.40 in (315.0 mm)	12.40 in (315.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	111.09 in (2,821.8 mm)	109.58 in (2,783.5 mm)	−1.51 in (−38.3 mm)	−1.36%
Revolutions per mile	570	578	+8	+1.38%

True speed with the speedometer calibrated for 315/70R18
Indicated	Actual on 315/70R18	Actual on 315/60R20	Difference
30 mph	30.0 mph	29.6 mph	−0.4 mph
60 mph	60.0 mph	59.2 mph	−0.8 mph
75 mph	75.0 mph	74.0 mph	−1.0 mph

Ride-height change: 315/60R20 lowers the vehicle by 0.24 in (6.1 mm) at the axle (half the −12.2 mm diameter difference).

315/60R20 is 0.48 in shorter than 315/70R18 and the same width, a 1.36% difference in overall diameter.

With the speedometer calibrated for 315/70R18, an indicated 60 mph is actually 59.2 mph on 315/60R20, and revolutions per mile change from 570 to 578.

315/60R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 315/70R18.

Size pages

Other comparisons with 315/70R18

Other comparisons with 315/60R20

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