265/40R17 vs 235/40R18

235/40R18 is 0.06 in (1.4 mm) taller than 265/40R17, a +0.22% diameter difference. With the speedometer set for 265/40R17, an indicated 60 mph is 60.1 mph on 235/40R18.

Is this a safe swap?

Diameter difference +0.22%: under 1%

By overall diameter alone, 235/40R18 is a near match for 265/40R17; 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.

265/40R17 vs 235/40R18 differences
Measurement	265/40R17	235/40R18	Difference	% diff
Overall diameter	25.35 in (643.8 mm)	25.40 in (645.2 mm)	+0.06 in (+1.4 mm)	+0.22%
Sidewall height	4.17 in (106.0 mm)	3.70 in (94.0 mm)	−0.47 in (−12.0 mm)	−11.32%
Section width	10.43 in (265.0 mm)	9.25 in (235.0 mm)	−1.18 in (−30.0 mm)	−11.32%
Circumference	79.63 in (2,022.6 mm)	79.80 in (2,027.0 mm)	+0.17 in (+4.4 mm)	+0.22%
Revolutions per mile	796	794	−2	−0.22%

True speed with the speedometer calibrated for 265/40R17
Indicated	Actual on 265/40R17	Actual on 235/40R18	Difference
30 mph	30.0 mph	30.1 mph	+0.1 mph
60 mph	60.0 mph	60.1 mph	+0.1 mph
75 mph	75.0 mph	75.2 mph	+0.2 mph

Ride-height change: 235/40R18 raises the vehicle by 0.03 in (0.7 mm) at the axle (half the +1.4 mm diameter difference).

235/40R18 is 0.06 in taller than 265/40R17 and 30 mm narrower, a 0.22% difference in overall diameter.

With the speedometer calibrated for 265/40R17, an indicated 60 mph is actually 60.1 mph on 235/40R18, and revolutions per mile change from 796 to 794.

235/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 265/40R17.

Size pages

Other comparisons with 265/40R17

Other comparisons with 235/40R18

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