305/40R20 vs 315/35R21

315/35R21 is 0.07 in (1.9 mm) taller than 305/40R20, a +0.25% diameter difference. With the speedometer set for 305/40R20, an indicated 60 mph is 60.2 mph on 315/35R21.

Is this a safe swap?

Diameter difference +0.25%: under 1%

By overall diameter alone, 315/35R21 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 315/35R21 differences
Measurement	305/40R20	315/35R21	Difference	% diff
Overall diameter	29.61 in (752.0 mm)	29.68 in (753.9 mm)	+0.07 in (+1.9 mm)	+0.25%
Sidewall height	4.80 in (122.0 mm)	4.34 in (110.3 mm)	−0.46 in (−11.7 mm)	−9.63%
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)	93.25 in (2,368.4 mm)	+0.24 in (+6.0 mm)	+0.25%
Revolutions per mile	681	679	−2	−0.25%

True speed with the speedometer calibrated for 305/40R20
Indicated	Actual on 305/40R20	Actual on 315/35R21	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.2 mph	+0.2 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 315/35R21 raises the vehicle by 0.04 in (0.9 mm) at the axle (half the +1.9 mm diameter difference).

315/35R21 is 0.07 in taller than 305/40R20 and 10 mm wider, a 0.25% difference in overall diameter.

With the speedometer calibrated for 305/40R20, an indicated 60 mph is actually 60.2 mph on 315/35R21, and revolutions per mile change from 681 to 679.

315/35R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 305/40R20.

Size pages

Other comparisons with 305/40R20

Other comparisons with 315/35R21

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