TireSizesFinder

285/40R20 vs 255/40R21

255/40R21 is 0.06 in (1.4 mm) taller than 285/40R20, a +0.19% diameter difference. With the speedometer set for 285/40R20, an indicated 60 mph is 60.1 mph on 255/40R21.

Is this a safe swap?

Diameter difference +0.19%: under 1%

By overall diameter alone, 255/40R21 is a near match for 285/40R20; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 285/40R20 (28.98 in) vs 255/40R21 (29.03 in)285/40R2028.98 in × 11.22 in255/40R2129.03 in × 10.04 in
Side profile and tread width, drawn to the same scale.
285/40R20 vs 255/40R21 differences
Measurement285/40R20255/40R21Difference% diff
Overall diameter28.98 in (736.0 mm)29.03 in (737.4 mm)+0.06 in (+1.4 mm)+0.19%
Sidewall height4.49 in (114.0 mm)4.02 in (102.0 mm)−0.47 in (−12.0 mm)−10.53%
Section width11.22 in (285.0 mm)10.04 in (255.0 mm)−1.18 in (−30.0 mm)−10.53%
Circumference91.03 in (2,312.2 mm)91.21 in (2,316.6 mm)+0.17 in (+4.4 mm)+0.19%
Revolutions per mile696695−1−0.19%
True speed with the speedometer calibrated for 285/40R20
IndicatedActual on 285/40R20Actual on 255/40R21Difference
30 mph30.0 mph30.1 mph+0.1 mph
60 mph60.0 mph60.1 mph+0.1 mph
75 mph75.0 mph75.1 mph+0.1 mph

Ride-height change: 255/40R21 raises the vehicle by 0.03 in (0.7 mm) at the axle (half the +1.4 mm diameter difference).

255/40R21 is 0.06 in taller than 285/40R20 and 30 mm narrower, a 0.19% difference in overall diameter.

With the speedometer calibrated for 285/40R20, an indicated 60 mph is actually 60.1 mph on 255/40R21, and revolutions per mile change from 696 to 695.

255/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 285/40R20.

Size pages

Other comparisons with 285/40R20

Other comparisons with 255/40R21

Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.