285/75R18 vs 315/70R18

315/70R18 is 0.53 in (13.5 mm) taller than 285/75R18, a +1.53% diameter difference. With the speedometer set for 285/75R18, an indicated 60 mph is 60.9 mph on 315/70R18.

Is this a safe swap?

Diameter difference +1.53%: within the ±3% guideline

By overall diameter alone, 315/70R18 falls inside the ±3% range commonly used as a guideline when replacing 285/75R18. 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 315/70R18 differences
Measurement	285/75R18	315/70R18	Difference	% diff
Overall diameter	34.83 in (884.7 mm)	35.36 in (898.2 mm)	+0.53 in (+13.5 mm)	+1.53%
Sidewall height	8.42 in (213.8 mm)	8.68 in (220.5 mm)	+0.27 in (+6.8 mm)	+3.16%
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)	111.09 in (2,821.8 mm)	+1.67 in (+42.4 mm)	+1.53%
Revolutions per mile	579	570	−9	−1.50%

True speed with the speedometer calibrated for 285/75R18
Indicated	Actual on 285/75R18	Actual on 315/70R18	Difference
30 mph	30.0 mph	30.5 mph	+0.5 mph
60 mph	60.0 mph	60.9 mph	+0.9 mph
75 mph	75.0 mph	76.1 mph	+1.1 mph

Ride-height change: 315/70R18 raises the vehicle by 0.27 in (6.8 mm) at the axle (half the +13.5 mm diameter difference).

315/70R18 is 0.53 in taller than 285/75R18 and 30 mm wider, a 1.53% difference in overall diameter.

With the speedometer calibrated for 285/75R18, an indicated 60 mph is actually 60.9 mph on 315/70R18, and revolutions per mile change from 579 to 570.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 285/75R18

Other comparisons with 315/70R18

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