235/55R18 vs 265/40R20

265/40R20 is 0.17 in (4.3 mm) taller than 235/55R18, a +0.60% diameter difference. With the speedometer set for 235/55R18, an indicated 60 mph is 60.4 mph on 265/40R20.

Is this a safe swap?

Diameter difference +0.60%: under 1%

By overall diameter alone, 265/40R20 is a near match for 235/55R18; the speedometer changes by less than 1%. 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.

235/55R18 vs 265/40R20 differences
Measurement	235/55R18	265/40R20	Difference	% diff
Overall diameter	28.18 in (715.7 mm)	28.35 in (720.0 mm)	+0.17 in (+4.3 mm)	+0.60%
Sidewall height	5.09 in (129.3 mm)	4.17 in (106.0 mm)	−0.92 in (−23.2 mm)	−17.99%
Section width	9.25 in (235.0 mm)	10.43 in (265.0 mm)	+1.18 in (+30.0 mm)	+12.77%
Circumference	88.52 in (2,248.4 mm)	89.05 in (2,261.9 mm)	+0.53 in (+13.5 mm)	+0.60%
Revolutions per mile	716	711	−4	−0.60%

True speed with the speedometer calibrated for 235/55R18
Indicated	Actual on 235/55R18	Actual on 265/40R20	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.4 mph	+0.4 mph
75 mph	75.0 mph	75.5 mph	+0.5 mph

Ride-height change: 265/40R20 raises the vehicle by 0.08 in (2.1 mm) at the axle (half the +4.3 mm diameter difference).

265/40R20 is 0.17 in taller than 235/55R18 and 30 mm wider, a 0.60% difference in overall diameter.

With the speedometer calibrated for 235/55R18, an indicated 60 mph is actually 60.4 mph on 265/40R20, and revolutions per mile change from 716 to 711.

265/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/55R18.

Size pages

Other comparisons with 235/55R18

Other comparisons with 265/40R20

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