285/35R20 vs 275/30R21

275/30R21 is 0.36 in (9.1 mm) shorter than 285/35R20, a −1.29% diameter difference. With the speedometer set for 285/35R20, an indicated 60 mph is 59.2 mph on 275/30R21.

Is this a safe swap?

Diameter difference −1.29%: within the ±3% guideline

By overall diameter alone, 275/30R21 falls inside the ±3% range commonly used as a guideline when replacing 285/35R20. 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.

285/35R20 vs 275/30R21 differences
Measurement	285/35R20	275/30R21	Difference	% diff
Overall diameter	27.85 in (707.5 mm)	27.50 in (698.4 mm)	−0.36 in (−9.1 mm)	−1.29%
Sidewall height	3.93 in (99.8 mm)	3.25 in (82.5 mm)	−0.68 in (−17.2 mm)	−17.29%
Section width	11.22 in (285.0 mm)	10.83 in (275.0 mm)	−0.39 in (−10.0 mm)	−3.51%
Circumference	87.51 in (2,222.7 mm)	86.38 in (2,194.1 mm)	−1.13 in (−28.6 mm)	−1.29%
Revolutions per mile	724	733	+9	+1.30%

True speed with the speedometer calibrated for 285/35R20
Indicated	Actual on 285/35R20	Actual on 275/30R21	Difference
30 mph	30.0 mph	29.6 mph	−0.4 mph
60 mph	60.0 mph	59.2 mph	−0.8 mph
75 mph	75.0 mph	74.0 mph	−1.0 mph

Ride-height change: 275/30R21 lowers the vehicle by 0.18 in (4.6 mm) at the axle (half the −9.1 mm diameter difference).

275/30R21 is 0.36 in shorter than 285/35R20 and 10 mm narrower, a 1.29% difference in overall diameter.

With the speedometer calibrated for 285/35R20, an indicated 60 mph is actually 59.2 mph on 275/30R21, and revolutions per mile change from 724 to 733.

275/30R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 285/35R20.

Size pages

Other comparisons with 285/35R20

Other comparisons with 275/30R21

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