265/65R18 vs 305/50R20

305/50R20 is 0.44 in (11.3 mm) taller than 265/65R18, a +1.41% diameter difference. With the speedometer set for 265/65R18, an indicated 60 mph is 60.8 mph on 305/50R20.

Is this a safe swap?

Diameter difference +1.41%: within the ±3% guideline

By overall diameter alone, 305/50R20 falls inside the ±3% range commonly used as a guideline when replacing 265/65R18. 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.

265/65R18 vs 305/50R20 differences
Measurement	265/65R18	305/50R20	Difference	% diff
Overall diameter	31.56 in (801.7 mm)	32.01 in (813.0 mm)	+0.44 in (+11.3 mm)	+1.41%
Sidewall height	6.78 in (172.3 mm)	6.00 in (152.5 mm)	−0.78 in (−19.7 mm)	−11.47%
Section width	10.43 in (265.0 mm)	12.01 in (305.0 mm)	+1.57 in (+40.0 mm)	+15.09%
Circumference	99.16 in (2,518.6 mm)	100.56 in (2,554.1 mm)	+1.40 in (+35.5 mm)	+1.41%
Revolutions per mile	639	630	−9	−1.39%

True speed with the speedometer calibrated for 265/65R18
Indicated	Actual on 265/65R18	Actual on 305/50R20	Difference
30 mph	30.0 mph	30.4 mph	+0.4 mph
60 mph	60.0 mph	60.8 mph	+0.8 mph
75 mph	75.0 mph	76.1 mph	+1.1 mph

Ride-height change: 305/50R20 raises the vehicle by 0.22 in (5.6 mm) at the axle (half the +11.3 mm diameter difference).

305/50R20 is 0.44 in taller than 265/65R18 and 40 mm wider, a 1.41% difference in overall diameter.

With the speedometer calibrated for 265/65R18, an indicated 60 mph is actually 60.8 mph on 305/50R20, and revolutions per mile change from 639 to 630.

305/50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 265/65R18.

Size pages

Other comparisons with 265/65R18

Other comparisons with 305/50R20

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