TireSizesFinder

285/40R20 vs 245/40R21

245/40R21 is 0.26 in (6.6 mm) shorter than 285/40R20, a −0.90% diameter difference. With the speedometer set for 285/40R20, an indicated 60 mph is 59.5 mph on 245/40R21.

Is this a safe swap?

Diameter difference −0.90%: under 1%

By overall diameter alone, 245/40R21 is a near match for 285/40R20; 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: 285/40R20 (28.98 in) vs 245/40R21 (28.72 in)285/40R2028.98 in × 11.22 in245/40R2128.72 in × 9.65 in
Side profile and tread width, drawn to the same scale.
285/40R20 vs 245/40R21 differences
Measurement285/40R20245/40R21Difference% diff
Overall diameter28.98 in (736.0 mm)28.72 in (729.4 mm)−0.26 in (−6.6 mm)−0.90%
Sidewall height4.49 in (114.0 mm)3.86 in (98.0 mm)−0.63 in (−16.0 mm)−14.04%
Section width11.22 in (285.0 mm)9.65 in (245.0 mm)−1.57 in (−40.0 mm)−14.04%
Circumference91.03 in (2,312.2 mm)90.22 in (2,291.5 mm)−0.82 in (−20.7 mm)−0.90%
Revolutions per mile696702+6+0.90%
True speed with the speedometer calibrated for 285/40R20
IndicatedActual on 285/40R20Actual on 245/40R21Difference
30 mph30.0 mph29.7 mph−0.3 mph
60 mph60.0 mph59.5 mph−0.5 mph
75 mph75.0 mph74.3 mph−0.7 mph

Ride-height change: 245/40R21 lowers the vehicle by 0.13 in (3.3 mm) at the axle (half the −6.6 mm diameter difference).

245/40R21 is 0.26 in shorter than 285/40R20 and 40 mm narrower, a 0.90% difference in overall diameter.

With the speedometer calibrated for 285/40R20, an indicated 60 mph is actually 59.5 mph on 245/40R21, and revolutions per mile change from 696 to 702.

245/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 285/40R20.

Size pages

Other comparisons with 285/40R20

Other comparisons with 245/40R21

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