265/35R18 vs 235/40R18

235/40R18 is 0.10 in (2.5 mm) taller than 265/35R18, a +0.39% diameter difference. With the speedometer set for 265/35R18, an indicated 60 mph is 60.2 mph on 235/40R18.

Is this a safe swap?

Diameter difference +0.39%: under 1%

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

265/35R18 vs 235/40R18 differences
Measurement	265/35R18	235/40R18	Difference	% diff
Overall diameter	25.30 in (642.7 mm)	25.40 in (645.2 mm)	+0.10 in (+2.5 mm)	+0.39%
Sidewall height	3.65 in (92.8 mm)	3.70 in (94.0 mm)	+0.05 in (+1.3 mm)	+1.35%
Section width	10.43 in (265.0 mm)	9.25 in (235.0 mm)	−1.18 in (−30.0 mm)	−11.32%
Circumference	79.49 in (2,019.1 mm)	79.80 in (2,027.0 mm)	+0.31 in (+7.9 mm)	+0.39%
Revolutions per mile	797	794	−3	−0.39%

True speed with the speedometer calibrated for 265/35R18
Indicated	Actual on 265/35R18	Actual on 235/40R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.2 mph	+0.2 mph
75 mph	75.0 mph	75.3 mph	+0.3 mph

Ride-height change: 235/40R18 raises the vehicle by 0.05 in (1.3 mm) at the axle (half the +2.5 mm diameter difference).

235/40R18 is 0.10 in taller than 265/35R18 and 30 mm narrower, a 0.39% difference in overall diameter.

With the speedometer calibrated for 265/35R18, an indicated 60 mph is actually 60.2 mph on 235/40R18, and revolutions per mile change from 797 to 794.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 265/35R18

Other comparisons with 235/40R18

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