245/40R20 vs 275/30R21

275/30R21 is 0.22 in (5.6 mm) shorter than 245/40R20, a −0.80% diameter difference. With the speedometer set for 245/40R20, an indicated 60 mph is 59.5 mph on 275/30R21.

Is this a safe swap?

Diameter difference −0.80%: under 1%

By overall diameter alone, 275/30R21 is a near match for 245/40R20; 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.

245/40R20 vs 275/30R21 differences
Measurement	245/40R20	275/30R21	Difference	% diff
Overall diameter	27.72 in (704.0 mm)	27.50 in (698.4 mm)	−0.22 in (−5.6 mm)	−0.80%
Sidewall height	3.86 in (98.0 mm)	3.25 in (82.5 mm)	−0.61 in (−15.5 mm)	−15.82%
Section width	9.65 in (245.0 mm)	10.83 in (275.0 mm)	+1.18 in (+30.0 mm)	+12.24%
Circumference	87.07 in (2,211.7 mm)	86.38 in (2,194.1 mm)	−0.69 in (−17.6 mm)	−0.80%
Revolutions per mile	728	733	+6	+0.80%

True speed with the speedometer calibrated for 245/40R20
Indicated	Actual on 245/40R20	Actual on 275/30R21	Difference
30 mph	30.0 mph	29.8 mph	−0.2 mph
60 mph	60.0 mph	59.5 mph	−0.5 mph
75 mph	75.0 mph	74.4 mph	−0.6 mph

Ride-height change: 275/30R21 lowers the vehicle by 0.11 in (2.8 mm) at the axle (half the −5.6 mm diameter difference).

275/30R21 is 0.22 in shorter than 245/40R20 and 30 mm wider, a 0.80% difference in overall diameter.

With the speedometer calibrated for 245/40R20, an indicated 60 mph is actually 59.5 mph on 275/30R21, and revolutions per mile change from 728 to 733.

275/30R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 245/40R20.

Size pages

Other comparisons with 245/40R20

Other comparisons with 275/30R21

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