325/30R21 vs 285/35R21

285/35R21 is 0.18 in (4.5 mm) taller than 325/30R21, a +0.62% diameter difference. With the speedometer set for 325/30R21, an indicated 60 mph is 60.4 mph on 285/35R21.

Is this a safe swap?

Diameter difference +0.62%: under 1%

By overall diameter alone, 285/35R21 is a near match for 325/30R21; 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.

325/30R21 vs 285/35R21 differences
Measurement	325/30R21	285/35R21	Difference	% diff
Overall diameter	28.68 in (728.4 mm)	28.85 in (732.9 mm)	+0.18 in (+4.5 mm)	+0.62%
Sidewall height	3.84 in (97.5 mm)	3.93 in (99.8 mm)	+0.09 in (+2.3 mm)	+2.31%
Section width	12.80 in (325.0 mm)	11.22 in (285.0 mm)	−1.57 in (−40.0 mm)	−12.31%
Circumference	90.09 in (2,288.3 mm)	90.65 in (2,302.5 mm)	+0.56 in (+14.1 mm)	+0.62%
Revolutions per mile	703	699	−4	−0.61%

True speed with the speedometer calibrated for 325/30R21
Indicated	Actual on 325/30R21	Actual on 285/35R21	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 285/35R21 raises the vehicle by 0.09 in (2.3 mm) at the axle (half the +4.5 mm diameter difference).

285/35R21 is 0.18 in taller than 325/30R21 and 40 mm narrower, a 0.62% difference in overall diameter.

With the speedometer calibrated for 325/30R21, an indicated 60 mph is actually 60.4 mph on 285/35R21, and revolutions per mile change from 703 to 699.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 325/30R21

Other comparisons with 285/35R21

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