TireSizesFinder

255/50R20 vs 285/40R21

285/40R21 is 0.06 in (1.6 mm) shorter than 255/50R20, a −0.21% diameter difference. With the speedometer set for 255/50R20, an indicated 60 mph is 59.9 mph on 285/40R21.

Is this a safe swap?

Diameter difference −0.21%: under 1%

By overall diameter alone, 285/40R21 is a near match for 255/50R20; 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: 255/50R20 (30.04 in) vs 285/40R21 (29.98 in)255/50R2030.04 in × 10.04 in285/40R2129.98 in × 11.22 in
Side profile and tread width, drawn to the same scale.
255/50R20 vs 285/40R21 differences
Measurement255/50R20285/40R21Difference% diff
Overall diameter30.04 in (763.0 mm)29.98 in (761.4 mm)−0.06 in (−1.6 mm)−0.21%
Sidewall height5.02 in (127.5 mm)4.49 in (114.0 mm)−0.53 in (−13.5 mm)−10.59%
Section width10.04 in (255.0 mm)11.22 in (285.0 mm)+1.18 in (+30.0 mm)+11.76%
Circumference94.37 in (2,397.0 mm)94.17 in (2,392.0 mm)−0.20 in (−5.0 mm)−0.21%
Revolutions per mile671673+1+0.21%
True speed with the speedometer calibrated for 255/50R20
IndicatedActual on 255/50R20Actual on 285/40R21Difference
30 mph30.0 mph29.9 mph−0.1 mph
60 mph60.0 mph59.9 mph−0.1 mph
75 mph75.0 mph74.8 mph−0.2 mph

Ride-height change: 285/40R21 lowers the vehicle by 0.03 in (0.8 mm) at the axle (half the −1.6 mm diameter difference).

285/40R21 is 0.06 in shorter than 255/50R20 and 30 mm wider, a 0.21% difference in overall diameter.

With the speedometer calibrated for 255/50R20, an indicated 60 mph is actually 59.9 mph on 285/40R21, and revolutions per mile change from 671 to 673.

285/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/50R20.

Size pages

Other comparisons with 255/50R20

Other comparisons with 285/40R21

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