255/55R20 vs 275/45R21

275/45R21 is 0.30 in (7.6 mm) shorter than 255/55R20, a −0.96% diameter difference. With the speedometer set for 255/55R20, an indicated 60 mph is 59.4 mph on 275/45R21.

Is this a safe swap?

Diameter difference −0.96%: under 1%

By overall diameter alone, 275/45R21 is a near match for 255/55R20; 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.

255/55R20 vs 275/45R21 differences
Measurement	255/55R20	275/45R21	Difference	% diff
Overall diameter	31.04 in (788.5 mm)	30.74 in (780.9 mm)	−0.30 in (−7.6 mm)	−0.96%
Sidewall height	5.52 in (140.3 mm)	4.87 in (123.8 mm)	−0.65 in (−16.5 mm)	−11.76%
Section width	10.04 in (255.0 mm)	10.83 in (275.0 mm)	+0.79 in (+20.0 mm)	+7.84%
Circumference	97.53 in (2,477.1 mm)	96.59 in (2,453.3 mm)	−0.94 in (−23.9 mm)	−0.96%
Revolutions per mile	650	656	+6	+0.97%

True speed with the speedometer calibrated for 255/55R20
Indicated	Actual on 255/55R20	Actual on 275/45R21	Difference
30 mph	30.0 mph	29.7 mph	−0.3 mph
60 mph	60.0 mph	59.4 mph	−0.6 mph
75 mph	75.0 mph	74.3 mph	−0.7 mph

Ride-height change: 275/45R21 lowers the vehicle by 0.15 in (3.8 mm) at the axle (half the −7.6 mm diameter difference).

275/45R21 is 0.30 in shorter than 255/55R20 and 20 mm wider, a 0.96% difference in overall diameter.

With the speedometer calibrated for 255/55R20, an indicated 60 mph is actually 59.4 mph on 275/45R21, and revolutions per mile change from 650 to 656.

275/45R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/55R20.

Size pages

Other comparisons with 255/55R20

Other comparisons with 275/45R21

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