255/45R20 vs 255/40R21

255/40R21 is 0.00 in (0.1 mm) shorter than 255/45R20, a −0.01% diameter difference. With the speedometer set for 255/45R20, an indicated 60 mph is 60.0 mph on 255/40R21.

Is this a safe swap?

Diameter difference −0.01%: under 1%

By overall diameter alone, 255/40R21 is a near match for 255/45R20; 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.

255/45R20 vs 255/40R21 differences
Measurement	255/45R20	255/40R21	Difference	% diff
Overall diameter	29.04 in (737.5 mm)	29.03 in (737.4 mm)	0.00 in (−0.1 mm)	−0.01%
Sidewall height	4.52 in (114.8 mm)	4.02 in (102.0 mm)	−0.50 in (−12.7 mm)	−11.11%
Section width	10.04 in (255.0 mm)	10.04 in (255.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	91.22 in (2,316.9 mm)	91.21 in (2,316.6 mm)	−0.01 in (−0.3 mm)	−0.01%
Revolutions per mile	695	695	0	+0.01%

True speed with the speedometer calibrated for 255/45R20
Indicated	Actual on 255/45R20	Actual on 255/40R21
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 255/40R21 lowers the vehicle by 0.00 in (0.1 mm) at the axle (half the −0.1 mm diameter difference).

255/40R21 is the same height as 255/45R20 and the same width, a 0.01% difference in overall diameter.

With the speedometer calibrated for 255/45R20, an indicated 60 mph is actually 60.0 mph on 255/40R21, and revolutions per mile change from 695 to 695.

255/40R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/45R20.

Size pages

Other comparisons with 255/45R20

Other comparisons with 255/40R21

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