275/45R21 vs 255/50R21

255/50R21 is 0.30 in (7.5 mm) taller than 275/45R21, a +0.96% diameter difference. With the speedometer set for 275/45R21, an indicated 60 mph is 60.6 mph on 255/50R21.

Is this a safe swap?

Diameter difference +0.96%: under 1%

By overall diameter alone, 255/50R21 is a near match for 275/45R21; 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.

275/45R21 vs 255/50R21 differences
Measurement	275/45R21	255/50R21	Difference	% diff
Overall diameter	30.74 in (780.9 mm)	31.04 in (788.4 mm)	+0.30 in (+7.5 mm)	+0.96%
Sidewall height	4.87 in (123.8 mm)	5.02 in (127.5 mm)	+0.15 in (+3.8 mm)	+3.03%
Section width	10.83 in (275.0 mm)	10.04 in (255.0 mm)	−0.79 in (−20.0 mm)	−7.27%
Circumference	96.59 in (2,453.3 mm)	97.51 in (2,476.8 mm)	+0.93 in (+23.6 mm)	+0.96%
Revolutions per mile	656	650	−6	−0.95%

True speed with the speedometer calibrated for 275/45R21
Indicated	Actual on 275/45R21	Actual on 255/50R21	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.6 mph	+0.6 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 255/50R21 raises the vehicle by 0.15 in (3.8 mm) at the axle (half the +7.5 mm diameter difference).

255/50R21 is 0.30 in taller than 275/45R21 and 20 mm narrower, a 0.96% difference in overall diameter.

With the speedometer calibrated for 275/45R21, an indicated 60 mph is actually 60.6 mph on 255/50R21, and revolutions per mile change from 656 to 650.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 275/45R21

Other comparisons with 255/50R21

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