TireSizesFinder

285/50R20 vs 255/50R21

255/50R21 is 0.18 in (4.6 mm) shorter than 285/50R20, a −0.58% diameter difference. With the speedometer set for 285/50R20, an indicated 60 mph is 59.7 mph on 255/50R21.

Is this a safe swap?

Diameter difference −0.58%: under 1%

By overall diameter alone, 255/50R21 is a near match for 285/50R20; 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: 285/50R20 (31.22 in) vs 255/50R21 (31.04 in)285/50R2031.22 in × 11.22 in255/50R2131.04 in × 10.04 in
Side profile and tread width, drawn to the same scale.
285/50R20 vs 255/50R21 differences
Measurement285/50R20255/50R21Difference% diff
Overall diameter31.22 in (793.0 mm)31.04 in (788.4 mm)−0.18 in (−4.6 mm)−0.58%
Sidewall height5.61 in (142.5 mm)5.02 in (127.5 mm)−0.59 in (−15.0 mm)−10.53%
Section width11.22 in (285.0 mm)10.04 in (255.0 mm)−1.18 in (−30.0 mm)−10.53%
Circumference98.08 in (2,491.3 mm)97.51 in (2,476.8 mm)−0.57 in (−14.5 mm)−0.58%
Revolutions per mile646650+4+0.58%
True speed with the speedometer calibrated for 285/50R20
IndicatedActual on 285/50R20Actual on 255/50R21Difference
30 mph30.0 mph29.8 mph−0.2 mph
60 mph60.0 mph59.7 mph−0.3 mph
75 mph75.0 mph74.6 mph−0.4 mph

Ride-height change: 255/50R21 lowers the vehicle by 0.09 in (2.3 mm) at the axle (half the −4.6 mm diameter difference).

255/50R21 is 0.18 in shorter than 285/50R20 and 30 mm narrower, a 0.58% difference in overall diameter.

With the speedometer calibrated for 285/50R20, an indicated 60 mph is actually 59.7 mph on 255/50R21, and revolutions per mile change from 646 to 650.

255/50R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 285/50R20.

Size pages

Other comparisons with 285/50R20

Other comparisons with 255/50R21

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