315/75R17 vs 305/70R18

305/70R18 is 0.79 in (20.1 mm) shorter than 315/75R17, a −2.22% diameter difference. With the speedometer set for 315/75R17, an indicated 60 mph is 58.7 mph on 305/70R18.

Is this a safe swap?

Diameter difference −2.22%: within the ±3% guideline

By overall diameter alone, 305/70R18 falls inside the ±3% range commonly used as a guideline when replacing 315/75R17. 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.

315/75R17 vs 305/70R18 differences
Measurement	315/75R17	305/70R18	Difference	% diff
Overall diameter	35.60 in (904.3 mm)	34.81 in (884.2 mm)	−0.79 in (−20.1 mm)	−2.22%
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	111.85 in (2,840.9 mm)	109.36 in (2,777.8 mm)	−2.49 in (−63.1 mm)	−2.22%
Revolutions per mile	566	579	+13	+2.27%

True speed with the speedometer calibrated for 315/75R17
Indicated	Actual on 315/75R17	Actual on 305/70R18	Difference
30 mph	30.0 mph	29.3 mph	−0.7 mph
60 mph	60.0 mph	58.7 mph	−1.3 mph
75 mph	75.0 mph	73.3 mph	−1.7 mph

Ride-height change: 305/70R18 lowers the vehicle by 0.40 in (10.0 mm) at the axle (half the −20.1 mm diameter difference).

305/70R18 is 0.79 in shorter than 315/75R17 and 10 mm narrower, a 2.22% difference in overall diameter.

With the speedometer calibrated for 315/75R17, an indicated 60 mph is actually 58.7 mph on 305/70R18, and revolutions per mile change from 566 to 579.

305/70R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 315/75R17.

Size pages

Other comparisons with 315/75R17

Other comparisons with 305/70R18

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