315/70R17 vs 305/70R18

305/70R18 is 0.45 in (11.4 mm) taller than 315/70R17, a +1.31% diameter difference. With the speedometer set for 315/70R17, an indicated 60 mph is 60.8 mph on 305/70R18.

Is this a safe swap?

Diameter difference +1.31%: within the ±3% guideline

By overall diameter alone, 305/70R18 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 305/70R18 differences
Measurement	315/70R17	305/70R18	Difference	% diff
Overall diameter	34.36 in (872.8 mm)	34.81 in (884.2 mm)	+0.45 in (+11.4 mm)	+1.31%
Sidewall height	8.68 in (220.5 mm)	8.41 in (213.5 mm)	−0.28 in (−7.0 mm)	−3.17%
Section width	12.40 in (315.0 mm)	12.01 in (305.0 mm)	−0.39 in (−10.0 mm)	−3.17%
Circumference	107.95 in (2,742.0 mm)	109.36 in (2,777.8 mm)	+1.41 in (+35.8 mm)	+1.31%
Revolutions per mile	587	579	−8	−1.29%

True speed with the speedometer calibrated for 315/70R17
Indicated	Actual on 315/70R17	Actual on 305/70R18	Difference
30 mph	30.0 mph	30.4 mph	+0.4 mph
60 mph	60.0 mph	60.8 mph	+0.8 mph
75 mph	75.0 mph	76.0 mph	+1.0 mph

Ride-height change: 305/70R18 raises the vehicle by 0.22 in (5.7 mm) at the axle (half the +11.4 mm diameter difference).

305/70R18 is 0.45 in taller than 315/70R17 and 10 mm narrower, a 1.31% difference in overall diameter.

With the speedometer calibrated for 315/70R17, an indicated 60 mph is actually 60.8 mph on 305/70R18, and revolutions per mile change from 587 to 579.

305/70R18 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 305/70R18

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