TireSizesFinder

285/45R20 vs 285/40R21

285/40R21 is 0.12 in (3.1 mm) shorter than 285/45R20, a −0.41% diameter difference. With the speedometer set for 285/45R20, an indicated 60 mph is 59.8 mph on 285/40R21.

Is this a safe swap?

Diameter difference −0.41%: under 1%

By overall diameter alone, 285/40R21 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 285/40R21 (29.98 in)285/45R2030.10 in × 11.22 in285/40R2129.98 in × 11.22 in
Side profile and tread width, drawn to the same scale.
285/45R20 vs 285/40R21 differences
Measurement285/45R20285/40R21Difference% diff
Overall diameter30.10 in (764.5 mm)29.98 in (761.4 mm)−0.12 in (−3.1 mm)−0.41%
Sidewall height5.05 in (128.3 mm)4.49 in (114.0 mm)−0.56 in (−14.2 mm)−11.11%
Section width11.22 in (285.0 mm)11.22 in (285.0 mm)0.00 in (0.0 mm)0.00%
Circumference94.56 in (2,401.7 mm)94.17 in (2,392.0 mm)−0.38 in (−9.7 mm)−0.41%
Revolutions per mile670673+3+0.41%
True speed with the speedometer calibrated for 285/45R20
IndicatedActual on 285/45R20Actual on 285/40R21Difference
30 mph30.0 mph29.9 mph−0.1 mph
60 mph60.0 mph59.8 mph−0.2 mph
75 mph75.0 mph74.7 mph−0.3 mph

Ride-height change: 285/40R21 lowers the vehicle by 0.06 in (1.6 mm) at the axle (half the −3.1 mm diameter difference).

285/40R21 is 0.12 in shorter than 285/45R20 and the same width, a 0.41% difference in overall diameter.

With the speedometer calibrated for 285/45R20, an indicated 60 mph is actually 59.8 mph on 285/40R21, and revolutions per mile change from 670 to 673.

285/40R21 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 285/40R21

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