285/40R19 vs 255/35R21

255/35R21 is 0.05 in (1.3 mm) taller than 285/40R19, a +0.18% diameter difference. With the speedometer set for 285/40R19, an indicated 60 mph is 60.1 mph on 255/35R21.

Is this a safe swap?

Diameter difference +0.18%: under 1%

By overall diameter alone, 255/35R21 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 255/35R21 differences
Measurement	285/40R19	255/35R21	Difference	% diff
Overall diameter	27.98 in (710.6 mm)	28.03 in (711.9 mm)	+0.05 in (+1.3 mm)	+0.18%
Sidewall height	4.49 in (114.0 mm)	3.51 in (89.3 mm)	−0.97 in (−24.7 mm)	−21.71%
Section width	11.22 in (285.0 mm)	10.04 in (255.0 mm)	−1.18 in (−30.0 mm)	−10.53%
Circumference	87.89 in (2,232.4 mm)	88.05 in (2,236.5 mm)	+0.16 in (+4.1 mm)	+0.18%
Revolutions per mile	721	720	−1	−0.18%

True speed with the speedometer calibrated for 285/40R19
Indicated	Actual on 285/40R19	Actual on 255/35R21	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 255/35R21 raises the vehicle by 0.03 in (0.7 mm) at the axle (half the +1.3 mm diameter difference).

255/35R21 is 0.05 in taller than 285/40R19 and 30 mm narrower, a 0.18% difference in overall diameter.

With the speedometer calibrated for 285/40R19, an indicated 60 mph is actually 60.1 mph on 255/35R21, and revolutions per mile change from 721 to 720.

255/35R21 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 255/35R21

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