255/50R20 vs 265/45R21

265/45R21 is 0.35 in (8.9 mm) taller than 255/50R20, a +1.17% diameter difference. With the speedometer set for 255/50R20, an indicated 60 mph is 60.7 mph on 265/45R21.

Is this a safe swap?

Diameter difference +1.17%: within the ±3% guideline

By overall diameter alone, 265/45R21 falls inside the ±3% range commonly used as a guideline when replacing 255/50R20. 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.

255/50R20 vs 265/45R21 differences
Measurement	255/50R20	265/45R21	Difference	% diff
Overall diameter	30.04 in (763.0 mm)	30.39 in (771.9 mm)	+0.35 in (+8.9 mm)	+1.17%
Sidewall height	5.02 in (127.5 mm)	4.69 in (119.3 mm)	−0.32 in (−8.2 mm)	−6.47%
Section width	10.04 in (255.0 mm)	10.43 in (265.0 mm)	+0.39 in (+10.0 mm)	+3.92%
Circumference	94.37 in (2,397.0 mm)	95.47 in (2,425.0 mm)	+1.10 in (+28.0 mm)	+1.17%
Revolutions per mile	671	664	−8	−1.15%

True speed with the speedometer calibrated for 255/50R20
Indicated	Actual on 255/50R20	Actual on 265/45R21	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.7 mph	+0.7 mph
75 mph	75.0 mph	75.9 mph	+0.9 mph

Ride-height change: 265/45R21 raises the vehicle by 0.18 in (4.4 mm) at the axle (half the +8.9 mm diameter difference).

265/45R21 is 0.35 in taller than 255/50R20 and 10 mm wider, a 1.17% difference in overall diameter.

With the speedometer calibrated for 255/50R20, an indicated 60 mph is actually 60.7 mph on 265/45R21, and revolutions per mile change from 671 to 664.

265/45R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 255/50R20.

Size pages

Other comparisons with 255/50R20

Other comparisons with 265/45R21

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