305/50R20 vs 265/50R21

265/50R21 is 0.57 in (14.6 mm) shorter than 305/50R20, a −1.80% diameter difference. With the speedometer set for 305/50R20, an indicated 60 mph is 58.9 mph on 265/50R21.

Is this a safe swap?

Diameter difference −1.80%: within the ±3% guideline

By overall diameter alone, 265/50R21 falls inside the ±3% range commonly used as a guideline when replacing 305/50R20. 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.

305/50R20 vs 265/50R21 differences
Measurement	305/50R20	265/50R21	Difference	% diff
Overall diameter	32.01 in (813.0 mm)	31.43 in (798.4 mm)	−0.57 in (−14.6 mm)	−1.80%
Sidewall height	6.00 in (152.5 mm)	5.22 in (132.5 mm)	−0.79 in (−20.0 mm)	−13.11%
Section width	12.01 in (305.0 mm)	10.43 in (265.0 mm)	−1.57 in (−40.0 mm)	−13.11%
Circumference	100.56 in (2,554.1 mm)	98.75 in (2,508.2 mm)	−1.81 in (−45.9 mm)	−1.80%
Revolutions per mile	630	642	+12	+1.83%

True speed with the speedometer calibrated for 305/50R20
Indicated	Actual on 305/50R20	Actual on 265/50R21	Difference
30 mph	30.0 mph	29.5 mph	−0.5 mph
60 mph	60.0 mph	58.9 mph	−1.1 mph
75 mph	75.0 mph	73.7 mph	−1.3 mph

Ride-height change: 265/50R21 lowers the vehicle by 0.29 in (7.3 mm) at the axle (half the −14.6 mm diameter difference).

265/50R21 is 0.57 in shorter than 305/50R20 and 40 mm narrower, a 1.80% difference in overall diameter.

With the speedometer calibrated for 305/50R20, an indicated 60 mph is actually 58.9 mph on 265/50R21, and revolutions per mile change from 630 to 642.

265/50R21 mounts on a 21-inch wheel, a plus-1 fitment from the 20-inch wheel of 305/50R20.

Size pages

Other comparisons with 305/50R20

Other comparisons with 265/50R21

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