TireSizesFinder

305/50R20 vs 275/50R21

275/50R21 is 0.18 in (4.6 mm) shorter than 305/50R20, a −0.57% diameter difference. With the speedometer set for 305/50R20, an indicated 60 mph is 59.7 mph on 275/50R21.

Is this a safe swap?

Diameter difference −0.57%: under 1%

By overall diameter alone, 275/50R21 is a near match for 305/50R20; 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: 305/50R20 (32.01 in) vs 275/50R21 (31.83 in)305/50R2032.01 in × 12.01 in275/50R2131.83 in × 10.83 in
Side profile and tread width, drawn to the same scale.
305/50R20 vs 275/50R21 differences
Measurement305/50R20275/50R21Difference% diff
Overall diameter32.01 in (813.0 mm)31.83 in (808.4 mm)−0.18 in (−4.6 mm)−0.57%
Sidewall height6.00 in (152.5 mm)5.41 in (137.5 mm)−0.59 in (−15.0 mm)−9.84%
Section width12.01 in (305.0 mm)10.83 in (275.0 mm)−1.18 in (−30.0 mm)−9.84%
Circumference100.56 in (2,554.1 mm)99.99 in (2,539.7 mm)−0.57 in (−14.5 mm)−0.57%
Revolutions per mile630634+4+0.57%
True speed with the speedometer calibrated for 305/50R20
IndicatedActual on 305/50R20Actual on 275/50R21Difference
30 mph30.0 mph29.8 mph−0.2 mph
60 mph60.0 mph59.7 mph−0.3 mph
75 mph75.0 mph74.6 mph−0.4 mph

Ride-height change: 275/50R21 lowers the vehicle by 0.09 in (2.3 mm) at the axle (half the −4.6 mm diameter difference).

275/50R21 is 0.18 in shorter than 305/50R20 and 30 mm narrower, a 0.57% difference in overall diameter.

With the speedometer calibrated for 305/50R20, an indicated 60 mph is actually 59.7 mph on 275/50R21, and revolutions per mile change from 630 to 634.

275/50R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 305/50R20.

Size pages

Other comparisons with 305/50R20

Other comparisons with 275/50R21

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