285/40R19 vs 305/30R21

305/30R21 is 0.23 in (5.8 mm) taller than 285/40R19, a +0.82% diameter difference. With the speedometer set for 285/40R19, an indicated 60 mph is 60.5 mph on 305/30R21.

Is this a safe swap?

Diameter difference +0.82%: under 1%

By overall diameter alone, 305/30R21 is a near match for 285/40R19; 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.

285/40R19 vs 305/30R21 differences
Measurement	285/40R19	305/30R21	Difference	% diff
Overall diameter	27.98 in (710.6 mm)	28.20 in (716.4 mm)	+0.23 in (+5.8 mm)	+0.82%
Sidewall height	4.49 in (114.0 mm)	3.60 in (91.5 mm)	−0.89 in (−22.5 mm)	−19.74%
Section width	11.22 in (285.0 mm)	12.01 in (305.0 mm)	+0.79 in (+20.0 mm)	+7.02%
Circumference	87.89 in (2,232.4 mm)	88.61 in (2,250.6 mm)	+0.72 in (+18.2 mm)	+0.82%
Revolutions per mile	721	715	−6	−0.81%

True speed with the speedometer calibrated for 285/40R19
Indicated	Actual on 285/40R19	Actual on 305/30R21	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 305/30R21 raises the vehicle by 0.11 in (2.9 mm) at the axle (half the +5.8 mm diameter difference).

305/30R21 is 0.23 in taller than 285/40R19 and 20 mm wider, a 0.82% difference in overall diameter.

With the speedometer calibrated for 285/40R19, an indicated 60 mph is actually 60.5 mph on 305/30R21, and revolutions per mile change from 721 to 715.

305/30R21 mounts on a 21-inch wheel, a plus-2 fitment from the 19-inch wheel of 285/40R19.

Size pages

Other comparisons with 285/40R19

Other comparisons with 305/30R21

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