305/30R21 vs 325/30R21

325/30R21 is 0.47 in (12.0 mm) taller than 305/30R21, a +1.68% diameter difference. With the speedometer set for 305/30R21, an indicated 60 mph is 61.0 mph on 325/30R21.

Is this a safe swap?

Diameter difference +1.68%: within the ±3% guideline

By overall diameter alone, 325/30R21 falls inside the ±3% range commonly used as a guideline when replacing 305/30R21. 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.

305/30R21 vs 325/30R21 differences
Measurement	305/30R21	325/30R21	Difference	% diff
Overall diameter	28.20 in (716.4 mm)	28.68 in (728.4 mm)	+0.47 in (+12.0 mm)	+1.68%
Sidewall height	3.60 in (91.5 mm)	3.84 in (97.5 mm)	+0.24 in (+6.0 mm)	+6.56%
Section width	12.01 in (305.0 mm)	12.80 in (325.0 mm)	+0.79 in (+20.0 mm)	+6.56%
Circumference	88.61 in (2,250.6 mm)	90.09 in (2,288.3 mm)	+1.48 in (+37.7 mm)	+1.68%
Revolutions per mile	715	703	−12	−1.65%

True speed with the speedometer calibrated for 305/30R21
Indicated	Actual on 305/30R21	Actual on 325/30R21	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	76.3 mph	+1.3 mph

Ride-height change: 325/30R21 raises the vehicle by 0.24 in (6.0 mm) at the axle (half the +12.0 mm diameter difference).

325/30R21 is 0.47 in taller than 305/30R21 and 20 mm wider, a 1.68% difference in overall diameter.

With the speedometer calibrated for 305/30R21, an indicated 60 mph is actually 61.0 mph on 325/30R21, and revolutions per mile change from 715 to 703.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 305/30R21

Other comparisons with 325/30R21

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