275/35R20 vs 275/30R21

275/30R21 is 0.08 in (2.1 mm) shorter than 275/35R20, a −0.30% diameter difference. With the speedometer set for 275/35R20, an indicated 60 mph is 59.8 mph on 275/30R21.

Is this a safe swap?

Diameter difference −0.30%: under 1%

By overall diameter alone, 275/30R21 is a near match for 275/35R20; 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.

275/35R20 vs 275/30R21 differences
Measurement	275/35R20	275/30R21	Difference	% diff
Overall diameter	27.58 in (700.5 mm)	27.50 in (698.4 mm)	−0.08 in (−2.1 mm)	−0.30%
Sidewall height	3.79 in (96.3 mm)	3.25 in (82.5 mm)	−0.54 in (−13.7 mm)	−14.29%
Section width	10.83 in (275.0 mm)	10.83 in (275.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	86.64 in (2,200.7 mm)	86.38 in (2,194.1 mm)	−0.26 in (−6.6 mm)	−0.30%
Revolutions per mile	731	733	+2	+0.30%

True speed with the speedometer calibrated for 275/35R20
Indicated	Actual on 275/35R20	Actual on 275/30R21	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.8 mph	−0.2 mph
75 mph	75.0 mph	74.8 mph	−0.2 mph

Ride-height change: 275/30R21 lowers the vehicle by 0.04 in (1.1 mm) at the axle (half the −2.1 mm diameter difference).

275/30R21 is 0.08 in shorter than 275/35R20 and the same width, a 0.30% difference in overall diameter.

With the speedometer calibrated for 275/35R20, an indicated 60 mph is actually 59.8 mph on 275/30R21, and revolutions per mile change from 731 to 733.

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

Size pages

Other comparisons with 275/35R20

Other comparisons with 275/30R21

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