305/70R18 vs 315/70R18

315/70R18 is 0.55 in (14.0 mm) taller than 305/70R18, a +1.58% diameter difference. With the speedometer set for 305/70R18, an indicated 60 mph is 61.0 mph on 315/70R18.

Is this a safe swap?

Diameter difference +1.58%: within the ±3% guideline

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

305/70R18 vs 315/70R18 differences
Measurement	305/70R18	315/70R18	Difference	% diff
Overall diameter	34.81 in (884.2 mm)	35.36 in (898.2 mm)	+0.55 in (+14.0 mm)	+1.58%
Sidewall height	8.41 in (213.5 mm)	8.68 in (220.5 mm)	+0.28 in (+7.0 mm)	+3.28%
Section width	12.01 in (305.0 mm)	12.40 in (315.0 mm)	+0.39 in (+10.0 mm)	+3.28%
Circumference	109.36 in (2,777.8 mm)	111.09 in (2,821.8 mm)	+1.73 in (+44.0 mm)	+1.58%
Revolutions per mile	579	570	−9	−1.56%

True speed with the speedometer calibrated for 305/70R18
Indicated	Actual on 305/70R18	Actual on 315/70R18	Difference
30 mph	30.0 mph	30.5 mph	+0.5 mph
60 mph	60.0 mph	61.0 mph	+1.0 mph
75 mph	75.0 mph	76.2 mph	+1.2 mph

Ride-height change: 315/70R18 raises the vehicle by 0.28 in (7.0 mm) at the axle (half the +14.0 mm diameter difference).

315/70R18 is 0.55 in taller than 305/70R18 and 10 mm wider, a 1.58% difference in overall diameter.

With the speedometer calibrated for 305/70R18, an indicated 60 mph is actually 61.0 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 305/70R18

Other comparisons with 315/70R18

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