265/40R18 vs 235/35R20

235/35R20 is 0.13 in (3.3 mm) taller than 265/40R18, a +0.49% diameter difference. With the speedometer set for 265/40R18, an indicated 60 mph is 60.3 mph on 235/35R20.

Is this a safe swap?

Diameter difference +0.49%: under 1%

By overall diameter alone, 235/35R20 is a near match for 265/40R18; 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.

265/40R18 vs 235/35R20 differences
Measurement	265/40R18	235/35R20	Difference	% diff
Overall diameter	26.35 in (669.2 mm)	26.48 in (672.5 mm)	+0.13 in (+3.3 mm)	+0.49%
Sidewall height	4.17 in (106.0 mm)	3.24 in (82.3 mm)	−0.94 in (−23.7 mm)	−22.41%
Section width	10.43 in (265.0 mm)	9.25 in (235.0 mm)	−1.18 in (−30.0 mm)	−11.32%
Circumference	82.77 in (2,102.4 mm)	83.18 in (2,112.7 mm)	+0.41 in (+10.4 mm)	+0.49%
Revolutions per mile	765	762	−4	−0.49%

True speed with the speedometer calibrated for 265/40R18
Indicated	Actual on 265/40R18	Actual on 235/35R20	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.3 mph	+0.3 mph
75 mph	75.0 mph	75.4 mph	+0.4 mph

Ride-height change: 235/35R20 raises the vehicle by 0.06 in (1.6 mm) at the axle (half the +3.3 mm diameter difference).

235/35R20 is 0.13 in taller than 265/40R18 and 30 mm narrower, a 0.49% difference in overall diameter.

With the speedometer calibrated for 265/40R18, an indicated 60 mph is actually 60.3 mph on 235/35R20, and revolutions per mile change from 765 to 762.

235/35R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 265/40R18.

Size pages

Other comparisons with 265/40R18

Other comparisons with 235/35R20

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