275/35R21 vs 325/30R21

325/30R21 is 0.10 in (2.5 mm) taller than 275/35R21, a +0.34% diameter difference. With the speedometer set for 275/35R21, an indicated 60 mph is 60.2 mph on 325/30R21.

Is this a safe swap?

Diameter difference +0.34%: under 1%

By overall diameter alone, 325/30R21 is a near match for 275/35R21; 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.

275/35R21 vs 325/30R21 differences
Measurement	275/35R21	325/30R21	Difference	% diff
Overall diameter	28.58 in (725.9 mm)	28.68 in (728.4 mm)	+0.10 in (+2.5 mm)	+0.34%
Sidewall height	3.79 in (96.3 mm)	3.84 in (97.5 mm)	+0.05 in (+1.3 mm)	+1.30%
Section width	10.83 in (275.0 mm)	12.80 in (325.0 mm)	+1.97 in (+50.0 mm)	+18.18%
Circumference	89.78 in (2,280.5 mm)	90.09 in (2,288.3 mm)	+0.31 in (+7.9 mm)	+0.34%
Revolutions per mile	706	703	−2	−0.34%

True speed with the speedometer calibrated for 275/35R21
Indicated	Actual on 275/35R21	Actual on 325/30R21	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	75.3 mph	+0.3 mph

Ride-height change: 325/30R21 raises the vehicle by 0.05 in (1.3 mm) at the axle (half the +2.5 mm diameter difference).

325/30R21 is 0.10 in taller than 275/35R21 and 50 mm wider, a 0.34% difference in overall diameter.

With the speedometer calibrated for 275/35R21, an indicated 60 mph is actually 60.2 mph on 325/30R21, and revolutions per mile change from 706 to 703.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 275/35R21

Other comparisons with 325/30R21

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