265/45R20 vs 265/40R21

265/40R21 is 0.04 in (1.1 mm) shorter than 265/45R20, a −0.15% diameter difference. With the speedometer set for 265/45R20, an indicated 60 mph is 59.9 mph on 265/40R21.

Is this a safe swap?

Diameter difference −0.15%: under 1%

By overall diameter alone, 265/40R21 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 265/40R21 differences
Measurement	265/45R20	265/40R21	Difference	% diff
Overall diameter	29.39 in (746.5 mm)	29.35 in (745.4 mm)	−0.04 in (−1.1 mm)	−0.15%
Sidewall height	4.69 in (119.3 mm)	4.17 in (106.0 mm)	−0.52 in (−13.2 mm)	−11.11%
Section width	10.43 in (265.0 mm)	10.43 in (265.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	92.33 in (2,345.2 mm)	92.19 in (2,341.7 mm)	−0.14 in (−3.5 mm)	−0.15%
Revolutions per mile	686	687	+1	+0.15%

True speed with the speedometer calibrated for 265/45R20
Indicated	Actual on 265/45R20	Actual on 265/40R21	Difference
30 mph	30.0 mph	30.0 mph	0.0 mph
60 mph	60.0 mph	59.9 mph	−0.1 mph
75 mph	75.0 mph	74.9 mph	−0.1 mph

Ride-height change: 265/40R21 lowers the vehicle by 0.02 in (0.6 mm) at the axle (half the −1.1 mm diameter difference).

265/40R21 is 0.04 in shorter than 265/45R20 and the same width, a 0.15% difference in overall diameter.

With the speedometer calibrated for 265/45R20, an indicated 60 mph is actually 59.9 mph on 265/40R21, and revolutions per mile change from 686 to 687.

265/40R21 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 265/40R21

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