255/65R18 vs 265/55R20

265/55R20 is 0.43 in (10.8 mm) taller than 255/65R18, a +1.37% diameter difference. With the speedometer set for 255/65R18, an indicated 60 mph is 60.8 mph on 265/55R20.

Is this a safe swap?

Diameter difference +1.37%: within the ±3% guideline

By overall diameter alone, 265/55R20 falls inside the ±3% range commonly used as a guideline when replacing 255/65R18. 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.

255/65R18 vs 265/55R20 differences
Measurement	255/65R18	265/55R20	Difference	% diff
Overall diameter	31.05 in (788.7 mm)	31.48 in (799.5 mm)	+0.43 in (+10.8 mm)	+1.37%
Sidewall height	6.53 in (165.8 mm)	5.74 in (145.8 mm)	−0.79 in (−20.0 mm)	−12.07%
Section width	10.04 in (255.0 mm)	10.43 in (265.0 mm)	+0.39 in (+10.0 mm)	+3.92%
Circumference	97.55 in (2,477.8 mm)	98.89 in (2,511.7 mm)	+1.34 in (+33.9 mm)	+1.37%
Revolutions per mile	650	641	−9	−1.35%

True speed with the speedometer calibrated for 255/65R18
Indicated	Actual on 255/65R18	Actual on 265/55R20	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	76.0 mph	+1.0 mph

Ride-height change: 265/55R20 raises the vehicle by 0.21 in (5.4 mm) at the axle (half the +10.8 mm diameter difference).

265/55R20 is 0.43 in taller than 255/65R18 and 10 mm wider, a 1.37% difference in overall diameter.

With the speedometer calibrated for 255/65R18, an indicated 60 mph is actually 60.8 mph on 265/55R20, and revolutions per mile change from 650 to 641.

265/55R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/65R18.

Size pages

Other comparisons with 255/65R18

Other comparisons with 265/55R20

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