255/45R20 vs 265/40R21

265/40R21 is 0.31 in (7.9 mm) taller than 255/45R20, a +1.07% diameter difference. With the speedometer set for 255/45R20, an indicated 60 mph is 60.6 mph on 265/40R21.

Is this a safe swap?

Diameter difference +1.07%: within the ±3% guideline

By overall diameter alone, 265/40R21 falls inside the ±3% range commonly used as a guideline when replacing 255/45R20. 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 265/40R21 differences
Measurement	255/45R20	265/40R21	Difference	% diff
Overall diameter	29.04 in (737.5 mm)	29.35 in (745.4 mm)	+0.31 in (+7.9 mm)	+1.07%
Sidewall height	4.52 in (114.8 mm)	4.17 in (106.0 mm)	−0.34 in (−8.7 mm)	−7.63%
Section width	10.04 in (255.0 mm)	10.43 in (265.0 mm)	+0.39 in (+10.0 mm)	+3.92%
Circumference	91.22 in (2,316.9 mm)	92.19 in (2,341.7 mm)	+0.98 in (+24.8 mm)	+1.07%
Revolutions per mile	695	687	−7	−1.06%

True speed with the speedometer calibrated for 255/45R20
Indicated	Actual on 255/45R20	Actual on 265/40R21	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	75.8 mph	+0.8 mph

Ride-height change: 265/40R21 raises the vehicle by 0.16 in (3.9 mm) at the axle (half the +7.9 mm diameter difference).

265/40R21 is 0.31 in taller than 255/45R20 and 10 mm wider, a 1.07% difference in overall diameter.

With the speedometer calibrated for 255/45R20, an indicated 60 mph is actually 60.6 mph on 265/40R21, and revolutions per mile change from 695 to 687.

265/40R21 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 265/40R21

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