265/45R20 vs 275/40R21

275/40R21 is 0.27 in (6.9 mm) taller than 265/45R20, a +0.92% diameter difference. With the speedometer set for 265/45R20, an indicated 60 mph is 60.6 mph on 275/40R21.

Is this a safe swap?

Diameter difference +0.92%: under 1%

By overall diameter alone, 275/40R21 is a near match for 265/45R20; 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.

265/45R20 vs 275/40R21 differences
Measurement	265/45R20	275/40R21	Difference	% diff
Overall diameter	29.39 in (746.5 mm)	29.66 in (753.4 mm)	+0.27 in (+6.9 mm)	+0.92%
Sidewall height	4.69 in (119.3 mm)	4.33 in (110.0 mm)	−0.36 in (−9.2 mm)	−7.76%
Section width	10.43 in (265.0 mm)	10.83 in (275.0 mm)	+0.39 in (+10.0 mm)	+3.77%
Circumference	92.33 in (2,345.2 mm)	93.18 in (2,366.9 mm)	+0.85 in (+21.7 mm)	+0.92%
Revolutions per mile	686	680	−6	−0.92%

True speed with the speedometer calibrated for 265/45R20
Indicated	Actual on 265/45R20	Actual on 275/40R21	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.6 mph	+0.6 mph
75 mph	75.0 mph	75.7 mph	+0.7 mph

Ride-height change: 275/40R21 raises the vehicle by 0.14 in (3.4 mm) at the axle (half the +6.9 mm diameter difference).

275/40R21 is 0.27 in taller than 265/45R20 and 10 mm wider, a 0.92% difference in overall diameter.

With the speedometer calibrated for 265/45R20, an indicated 60 mph is actually 60.6 mph on 275/40R21, and revolutions per mile change from 686 to 680.

275/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 265/45R20.

Size pages

Other comparisons with 265/45R20

Other comparisons with 275/40R21

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