TireSizesFinder

235/55R20 vs 285/40R21

285/40R21 is 0.20 in (5.1 mm) shorter than 235/55R20, a −0.67% diameter difference. With the speedometer set for 235/55R20, an indicated 60 mph is 59.6 mph on 285/40R21.

Is this a safe swap?

Diameter difference −0.67%: under 1%

By overall diameter alone, 285/40R21 is a near match for 235/55R20; 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: 235/55R20 (30.18 in) vs 285/40R21 (29.98 in)235/55R2030.18 in × 9.25 in285/40R2129.98 in × 11.22 in
Side profile and tread width, drawn to the same scale.
235/55R20 vs 285/40R21 differences
Measurement235/55R20285/40R21Difference% diff
Overall diameter30.18 in (766.5 mm)29.98 in (761.4 mm)−0.20 in (−5.1 mm)−0.67%
Sidewall height5.09 in (129.3 mm)4.49 in (114.0 mm)−0.60 in (−15.2 mm)−11.80%
Section width9.25 in (235.0 mm)11.22 in (285.0 mm)+1.97 in (+50.0 mm)+21.28%
Circumference94.80 in (2,408.0 mm)94.17 in (2,392.0 mm)−0.63 in (−16.0 mm)−0.67%
Revolutions per mile668673+4+0.67%
True speed with the speedometer calibrated for 235/55R20
IndicatedActual on 235/55R20Actual on 285/40R21Difference
30 mph30.0 mph29.8 mph−0.2 mph
60 mph60.0 mph59.6 mph−0.4 mph
75 mph75.0 mph74.5 mph−0.5 mph

Ride-height change: 285/40R21 lowers the vehicle by 0.10 in (2.6 mm) at the axle (half the −5.1 mm diameter difference).

285/40R21 is 0.20 in shorter than 235/55R20 and 50 mm wider, a 0.67% difference in overall diameter.

With the speedometer calibrated for 235/55R20, an indicated 60 mph is actually 59.6 mph on 285/40R21, and revolutions per mile change from 668 to 673.

285/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 235/55R20.

Size pages

Other comparisons with 235/55R20

Other comparisons with 285/40R21

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