285/35R21 vs 255/40R21

255/40R21 is 0.18 in (4.5 mm) taller than 285/35R21, a +0.61% diameter difference. With the speedometer set for 285/35R21, an indicated 60 mph is 60.4 mph on 255/40R21.

Is this a safe swap?

Diameter difference +0.61%: under 1%

By overall diameter alone, 255/40R21 is a near match for 285/35R21; 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.

285/35R21 vs 255/40R21 differences
Measurement	285/35R21	255/40R21	Difference	% diff
Overall diameter	28.85 in (732.9 mm)	29.03 in (737.4 mm)	+0.18 in (+4.5 mm)	+0.61%
Sidewall height	3.93 in (99.8 mm)	4.02 in (102.0 mm)	+0.09 in (+2.3 mm)	+2.26%
Section width	11.22 in (285.0 mm)	10.04 in (255.0 mm)	−1.18 in (−30.0 mm)	−10.53%
Circumference	90.65 in (2,302.5 mm)	91.21 in (2,316.6 mm)	+0.56 in (+14.1 mm)	+0.61%
Revolutions per mile	699	695	−4	−0.61%

True speed with the speedometer calibrated for 285/35R21
Indicated	Actual on 285/35R21	Actual on 255/40R21	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: 255/40R21 raises the vehicle by 0.09 in (2.3 mm) at the axle (half the +4.5 mm diameter difference).

255/40R21 is 0.18 in taller than 285/35R21 and 30 mm narrower, a 0.61% difference in overall diameter.

With the speedometer calibrated for 285/35R21, an indicated 60 mph is actually 60.4 mph on 255/40R21, and revolutions per mile change from 699 to 695.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 285/35R21

Other comparisons with 255/40R21

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