TireSizesFinder

255/45R20 vs 285/35R21

285/35R21 is 0.18 in (4.6 mm) shorter than 255/45R20, a −0.62% diameter difference. With the speedometer set for 255/45R20, an indicated 60 mph is 59.6 mph on 285/35R21.

Is this a safe swap?

Diameter difference −0.62%: under 1%

By overall diameter alone, 285/35R21 is a near match for 255/45R20; 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: 255/45R20 (29.04 in) vs 285/35R21 (28.85 in)255/45R2029.04 in × 10.04 in285/35R2128.85 in × 11.22 in
Side profile and tread width, drawn to the same scale.
255/45R20 vs 285/35R21 differences
Measurement255/45R20285/35R21Difference% diff
Overall diameter29.04 in (737.5 mm)28.85 in (732.9 mm)−0.18 in (−4.6 mm)−0.62%
Sidewall height4.52 in (114.8 mm)3.93 in (99.8 mm)−0.59 in (−15.0 mm)−13.07%
Section width10.04 in (255.0 mm)11.22 in (285.0 mm)+1.18 in (+30.0 mm)+11.76%
Circumference91.22 in (2,316.9 mm)90.65 in (2,302.5 mm)−0.57 in (−14.5 mm)−0.62%
Revolutions per mile695699+4+0.63%
True speed with the speedometer calibrated for 255/45R20
IndicatedActual on 255/45R20Actual on 285/35R21Difference
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: 285/35R21 lowers the vehicle by 0.09 in (2.3 mm) at the axle (half the −4.6 mm diameter difference).

285/35R21 is 0.18 in shorter than 255/45R20 and 30 mm wider, a 0.62% difference in overall diameter.

With the speedometer calibrated for 255/45R20, an indicated 60 mph is actually 59.6 mph on 285/35R21, and revolutions per mile change from 695 to 699.

285/35R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/45R20.

Size pages

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