285/55R20 vs 275/50R21

275/50R21 is 0.52 in (13.1 mm) shorter than 285/55R20, a −1.59% diameter difference. With the speedometer set for 285/55R20, an indicated 60 mph is 59.0 mph on 275/50R21.

Is this a safe swap?

Diameter difference −1.59%: within the ±3% guideline

By overall diameter alone, 275/50R21 falls inside the ±3% range commonly used as a guideline when replacing 285/55R20. 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.

285/55R20 vs 275/50R21 differences
Measurement	285/55R20	275/50R21	Difference	% diff
Overall diameter	32.34 in (821.5 mm)	31.83 in (808.4 mm)	−0.52 in (−13.1 mm)	−1.59%
Sidewall height	6.17 in (156.8 mm)	5.41 in (137.5 mm)	−0.76 in (−19.2 mm)	−12.28%
Section width	11.22 in (285.0 mm)	10.83 in (275.0 mm)	−0.39 in (−10.0 mm)	−3.51%
Circumference	101.61 in (2,580.8 mm)	99.99 in (2,539.7 mm)	−1.62 in (−41.2 mm)	−1.59%
Revolutions per mile	624	634	+10	+1.62%

True speed with the speedometer calibrated for 285/55R20
Indicated	Actual on 285/55R20	Actual on 275/50R21	Difference
30 mph	30.0 mph	29.5 mph	−0.5 mph
60 mph	60.0 mph	59.0 mph	−1.0 mph
75 mph	75.0 mph	73.8 mph	−1.2 mph

Ride-height change: 275/50R21 lowers the vehicle by 0.26 in (6.6 mm) at the axle (half the −13.1 mm diameter difference).

275/50R21 is 0.52 in shorter than 285/55R20 and 10 mm narrower, a 1.59% difference in overall diameter.

With the speedometer calibrated for 285/55R20, an indicated 60 mph is actually 59.0 mph on 275/50R21, and revolutions per mile change from 624 to 634.

275/50R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 285/55R20.

Size pages

Other comparisons with 285/55R20

Other comparisons with 275/50R21

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