TireSizesFinder

305/45R20 vs 255/50R21

255/50R21 is 0.23 in (5.9 mm) taller than 305/45R20, a +0.75% diameter difference. With the speedometer set for 305/45R20, an indicated 60 mph is 60.5 mph on 255/50R21.

Is this a safe swap?

Diameter difference +0.75%: under 1%

By overall diameter alone, 255/50R21 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 255/50R21 (31.04 in)305/45R2030.81 in × 12.01 in255/50R2131.04 in × 10.04 in
Side profile and tread width, drawn to the same scale.
305/45R20 vs 255/50R21 differences
Measurement305/45R20255/50R21Difference% diff
Overall diameter30.81 in (782.5 mm)31.04 in (788.4 mm)+0.23 in (+5.9 mm)+0.75%
Sidewall height5.40 in (137.3 mm)5.02 in (127.5 mm)−0.38 in (−9.7 mm)−7.10%
Section width12.01 in (305.0 mm)10.04 in (255.0 mm)−1.97 in (−50.0 mm)−16.39%
Circumference96.78 in (2,458.3 mm)97.51 in (2,476.8 mm)+0.73 in (+18.5 mm)+0.75%
Revolutions per mile655650−5−0.75%
True speed with the speedometer calibrated for 305/45R20
IndicatedActual on 305/45R20Actual on 255/50R21Difference
30 mph30.0 mph30.2 mph+0.2 mph
60 mph60.0 mph60.5 mph+0.5 mph
75 mph75.0 mph75.6 mph+0.6 mph

Ride-height change: 255/50R21 raises the vehicle by 0.12 in (2.9 mm) at the axle (half the +5.9 mm diameter difference).

255/50R21 is 0.23 in taller than 305/45R20 and 50 mm narrower, a 0.75% difference in overall diameter.

With the speedometer calibrated for 305/45R20, an indicated 60 mph is actually 60.5 mph on 255/50R21, and revolutions per mile change from 655 to 650.

255/50R21 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 255/50R21

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