TireSizesFinder

285/45R20 vs 255/45R21

255/45R21 is 0.06 in (1.6 mm) shorter than 285/45R20, a −0.21% diameter difference. With the speedometer set for 285/45R20, an indicated 60 mph is 59.9 mph on 255/45R21.

Is this a safe swap?

Diameter difference −0.21%: under 1%

By overall diameter alone, 255/45R21 is a near match for 285/45R20; 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: 285/45R20 (30.10 in) vs 255/45R21 (30.04 in)285/45R2030.10 in × 11.22 in255/45R2130.04 in × 10.04 in
Side profile and tread width, drawn to the same scale.
285/45R20 vs 255/45R21 differences
Measurement285/45R20255/45R21Difference% diff
Overall diameter30.10 in (764.5 mm)30.04 in (762.9 mm)−0.06 in (−1.6 mm)−0.21%
Sidewall height5.05 in (128.3 mm)4.52 in (114.8 mm)−0.53 in (−13.5 mm)−10.53%
Section width11.22 in (285.0 mm)10.04 in (255.0 mm)−1.18 in (−30.0 mm)−10.53%
Circumference94.56 in (2,401.7 mm)94.36 in (2,396.7 mm)−0.20 in (−5.0 mm)−0.21%
Revolutions per mile670671+1+0.21%
True speed with the speedometer calibrated for 285/45R20
IndicatedActual on 285/45R20Actual on 255/45R21Difference
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: 255/45R21 lowers the vehicle by 0.03 in (0.8 mm) at the axle (half the −1.6 mm diameter difference).

255/45R21 is 0.06 in shorter than 285/45R20 and 30 mm narrower, a 0.21% difference in overall diameter.

With the speedometer calibrated for 285/45R20, an indicated 60 mph is actually 59.9 mph on 255/45R21, and revolutions per mile change from 670 to 671.

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

Size pages

Other comparisons with 285/45R20

Other comparisons with 255/45R21

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