305/40R20 vs 315/30R22

315/30R22 is 0.17 in (4.2 mm) shorter than 305/40R20, a −0.56% diameter difference. With the speedometer set for 305/40R20, an indicated 60 mph is 59.7 mph on 315/30R22.

Is this a safe swap?

Diameter difference −0.56%: under 1%

By overall diameter alone, 315/30R22 is a near match for 305/40R20; 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.

305/40R20 vs 315/30R22 differences
Measurement	305/40R20	315/30R22	Difference	% diff
Overall diameter	29.61 in (752.0 mm)	29.44 in (747.8 mm)	−0.17 in (−4.2 mm)	−0.56%
Sidewall height	4.80 in (122.0 mm)	3.72 in (94.5 mm)	−1.08 in (−27.5 mm)	−22.54%
Section width	12.01 in (305.0 mm)	12.40 in (315.0 mm)	+0.39 in (+10.0 mm)	+3.28%
Circumference	93.01 in (2,362.5 mm)	92.49 in (2,349.3 mm)	−0.52 in (−13.2 mm)	−0.56%
Revolutions per mile	681	685	+4	+0.56%

True speed with the speedometer calibrated for 305/40R20
Indicated	Actual on 305/40R20	Actual on 315/30R22	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.7 mph	−0.3 mph
75 mph	75.0 mph	74.6 mph	−0.4 mph

Ride-height change: 315/30R22 lowers the vehicle by 0.08 in (2.1 mm) at the axle (half the −4.2 mm diameter difference).

315/30R22 is 0.17 in shorter than 305/40R20 and 10 mm wider, a 0.56% difference in overall diameter.

With the speedometer calibrated for 305/40R20, an indicated 60 mph is actually 59.7 mph on 315/30R22, and revolutions per mile change from 681 to 685.

315/30R22 mounts on a 22-inch wheel, a plus-2 fitment from the 20-inch wheel of 305/40R20.

Size pages

Other comparisons with 305/40R20

Other comparisons with 315/30R22

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