315/35R20 vs 325/30R21

325/30R21 is 0.00 in (0.1 mm) shorter than 315/35R20, a −0.01% diameter difference. With the speedometer set for 315/35R20, an indicated 60 mph is 60.0 mph on 325/30R21.

Is this a safe swap?

Diameter difference −0.01%: under 1%

By overall diameter alone, 325/30R21 is a near match for 315/35R20; 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.

315/35R20 vs 325/30R21 differences
Measurement	315/35R20	325/30R21	Difference	% diff
Overall diameter	28.68 in (728.5 mm)	28.68 in (728.4 mm)	0.00 in (−0.1 mm)	−0.01%
Sidewall height	4.34 in (110.3 mm)	3.84 in (97.5 mm)	−0.50 in (−12.7 mm)	−11.56%
Section width	12.40 in (315.0 mm)	12.80 in (325.0 mm)	+0.39 in (+10.0 mm)	+3.17%
Circumference	90.10 in (2,288.7 mm)	90.09 in (2,288.3 mm)	−0.01 in (−0.3 mm)	−0.01%
Revolutions per mile	703	703	0	+0.01%

True speed with the speedometer calibrated for 315/35R20
Indicated	Actual on 315/35R20	Actual on 325/30R21
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 325/30R21 lowers the vehicle by 0.00 in (0.1 mm) at the axle (half the −0.1 mm diameter difference).

325/30R21 is the same height as 315/35R20 and 10 mm wider, a 0.01% difference in overall diameter.

With the speedometer calibrated for 315/35R20, an indicated 60 mph is actually 60.0 mph on 325/30R21, and revolutions per mile change from 703 to 703.

325/30R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 315/35R20.

Size pages

Other comparisons with 315/35R20

Other comparisons with 325/30R21

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