TireSizesFinder

305/45R20 vs 285/45R21

285/45R21 is 0.29 in (7.4 mm) taller than 305/45R20, a +0.95% diameter difference. With the speedometer set for 305/45R20, an indicated 60 mph is 60.6 mph on 285/45R21.

Is this a safe swap?

Diameter difference +0.95%: under 1%

By overall diameter alone, 285/45R21 is a near match for 305/45R20; 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: 305/45R20 (30.81 in) vs 285/45R21 (31.10 in)305/45R2030.81 in × 12.01 in285/45R2131.10 in × 11.22 in
Side profile and tread width, drawn to the same scale.
305/45R20 vs 285/45R21 differences
Measurement305/45R20285/45R21Difference% diff
Overall diameter30.81 in (782.5 mm)31.10 in (789.9 mm)+0.29 in (+7.4 mm)+0.95%
Sidewall height5.40 in (137.3 mm)5.05 in (128.3 mm)−0.35 in (−9.0 mm)−6.56%
Section width12.01 in (305.0 mm)11.22 in (285.0 mm)−0.79 in (−20.0 mm)−6.56%
Circumference96.78 in (2,458.3 mm)97.70 in (2,481.5 mm)+0.92 in (+23.2 mm)+0.95%
Revolutions per mile655649−6−0.94%
True speed with the speedometer calibrated for 305/45R20
IndicatedActual on 305/45R20Actual on 285/45R21Difference
30 mph30.0 mph30.3 mph+0.3 mph
60 mph60.0 mph60.6 mph+0.6 mph
75 mph75.0 mph75.7 mph+0.7 mph

Ride-height change: 285/45R21 raises the vehicle by 0.15 in (3.7 mm) at the axle (half the +7.4 mm diameter difference).

285/45R21 is 0.29 in taller than 305/45R20 and 20 mm narrower, a 0.95% difference in overall diameter.

With the speedometer calibrated for 305/45R20, an indicated 60 mph is actually 60.6 mph on 285/45R21, and revolutions per mile change from 655 to 649.

285/45R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 305/45R20.

Size pages

Other comparisons with 305/45R20

Other comparisons with 285/45R21

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