275/50R20 vs 255/50R21

255/50R21 is 0.21 in (5.4 mm) taller than 275/50R20, a +0.69% diameter difference. With the speedometer set for 275/50R20, an indicated 60 mph is 60.4 mph on 255/50R21.

Is this a safe swap?

Diameter difference +0.69%: under 1%

By overall diameter alone, 255/50R21 is a near match for 275/50R20; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

275/50R20 vs 255/50R21 differences
Measurement	275/50R20	255/50R21	Difference	% diff
Overall diameter	30.83 in (783.0 mm)	31.04 in (788.4 mm)	+0.21 in (+5.4 mm)	+0.69%
Sidewall height	5.41 in (137.5 mm)	5.02 in (127.5 mm)	−0.39 in (−10.0 mm)	−7.27%
Section width	10.83 in (275.0 mm)	10.04 in (255.0 mm)	−0.79 in (−20.0 mm)	−7.27%
Circumference	96.85 in (2,459.9 mm)	97.51 in (2,476.8 mm)	+0.67 in (+17.0 mm)	+0.69%
Revolutions per mile	654	650	−4	−0.68%

True speed with the speedometer calibrated for 275/50R20
Indicated	Actual on 275/50R20	Actual on 255/50R21	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 255/50R21 raises the vehicle by 0.11 in (2.7 mm) at the axle (half the +5.4 mm diameter difference).

255/50R21 is 0.21 in taller than 275/50R20 and 20 mm narrower, a 0.69% difference in overall diameter.

With the speedometer calibrated for 275/50R20, an indicated 60 mph is actually 60.4 mph on 255/50R21, and revolutions per mile change from 654 to 650.

255/50R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 275/50R20.

Size pages

Other comparisons with 275/50R20

Other comparisons with 255/50R21

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.