315/75R16 vs 305/70R18

305/70R18 is 0.21 in (5.3 mm) taller than 315/75R16, a +0.60% diameter difference. With the speedometer set for 315/75R16, an indicated 60 mph is 60.4 mph on 305/70R18.

Is this a safe swap?

Diameter difference +0.60%: under 1%

By overall diameter alone, 305/70R18 is a near match for 315/75R16; 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.

315/75R16 vs 305/70R18 differences
Measurement	315/75R16	305/70R18	Difference	% diff
Overall diameter	34.60 in (878.9 mm)	34.81 in (884.2 mm)	+0.21 in (+5.3 mm)	+0.60%
Sidewall height	9.30 in (236.3 mm)	8.41 in (213.5 mm)	−0.90 in (−22.7 mm)	−9.63%
Section width	12.40 in (315.0 mm)	12.01 in (305.0 mm)	−0.39 in (−10.0 mm)	−3.17%
Circumference	108.71 in (2,761.1 mm)	109.36 in (2,777.8 mm)	+0.66 in (+16.7 mm)	+0.60%
Revolutions per mile	583	579	−3	−0.60%

True speed with the speedometer calibrated for 315/75R16
Indicated	Actual on 315/75R16	Actual on 305/70R18	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 305/70R18 raises the vehicle by 0.10 in (2.7 mm) at the axle (half the +5.3 mm diameter difference).

305/70R18 is 0.21 in taller than 315/75R16 and 10 mm narrower, a 0.60% difference in overall diameter.

With the speedometer calibrated for 315/75R16, an indicated 60 mph is actually 60.4 mph on 305/70R18, and revolutions per mile change from 583 to 579.

305/70R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 315/75R16.

Size pages

Other comparisons with 315/75R16

Other comparisons with 305/70R18

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