315/70R17 vs 315/70R18

315/70R18 is 1.00 in (25.4 mm) taller than 315/70R17, a +2.91% diameter difference. With the speedometer set for 315/70R17, an indicated 60 mph is 61.7 mph on 315/70R18.

Is this a safe swap?

Diameter difference +2.91%: within the ±3% guideline

By overall diameter alone, 315/70R18 falls inside the ±3% range commonly used as a guideline when replacing 315/70R17. 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.

315/70R17 vs 315/70R18 differences
Measurement	315/70R17	315/70R18	Difference	% diff
Overall diameter	34.36 in (872.8 mm)	35.36 in (898.2 mm)	+1.00 in (+25.4 mm)	+2.91%
Sidewall height	8.68 in (220.5 mm)	8.68 in (220.5 mm)	0.00 in (0.0 mm)	0.00%
Section width	12.40 in (315.0 mm)	12.40 in (315.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	107.95 in (2,742.0 mm)	111.09 in (2,821.8 mm)	+3.14 in (+79.8 mm)	+2.91%
Revolutions per mile	587	570	−17	−2.83%

True speed with the speedometer calibrated for 315/70R17
Indicated	Actual on 315/70R17	Actual on 315/70R18	Difference
30 mph	30.0 mph	30.9 mph	+0.9 mph
60 mph	60.0 mph	61.7 mph	+1.7 mph
75 mph	75.0 mph	77.2 mph	+2.2 mph

Ride-height change: 315/70R18 raises the vehicle by 0.50 in (12.7 mm) at the axle (half the +25.4 mm diameter difference).

315/70R18 is 1.00 in taller than 315/70R17 and the same width, a 2.91% difference in overall diameter.

With the speedometer calibrated for 315/70R17, an indicated 60 mph is actually 61.7 mph on 315/70R18, and revolutions per mile change from 587 to 570.

315/70R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 315/70R17.

Size pages

Other comparisons with 315/70R17

Other comparisons with 315/70R18

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