235/45R18 vs 265/40R18

265/40R18 is 0.02 in (0.5 mm) taller than 235/45R18, a +0.07% diameter difference. With the speedometer set for 235/45R18, an indicated 60 mph is 60.0 mph on 265/40R18.

Is this a safe swap?

Diameter difference +0.07%: under 1%

By overall diameter alone, 265/40R18 is a near match for 235/45R18; 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.

235/45R18 vs 265/40R18 differences
Measurement	235/45R18	265/40R18	Difference	% diff
Overall diameter	26.33 in (668.7 mm)	26.35 in (669.2 mm)	+0.02 in (+0.5 mm)	+0.07%
Sidewall height	4.16 in (105.8 mm)	4.17 in (106.0 mm)	+0.01 in (+0.3 mm)	+0.24%
Section width	9.25 in (235.0 mm)	10.43 in (265.0 mm)	+1.18 in (+30.0 mm)	+12.77%
Circumference	82.71 in (2,100.8 mm)	82.77 in (2,102.4 mm)	+0.06 in (+1.6 mm)	+0.07%
Revolutions per mile	766	765	−1	−0.07%

True speed with the speedometer calibrated for 235/45R18
Indicated	Actual on 235/45R18	Actual on 265/40R18	Difference
30 mph	30.0 mph	30.0 mph	0.0 mph
60 mph	60.0 mph	60.0 mph	0.0 mph
75 mph	75.0 mph	75.1 mph	+0.1 mph

Ride-height change: 265/40R18 raises the vehicle by 0.01 in (0.3 mm) at the axle (half the +0.5 mm diameter difference).

265/40R18 is 0.02 in taller than 235/45R18 and 30 mm wider, a 0.07% difference in overall diameter.

With the speedometer calibrated for 235/45R18, an indicated 60 mph is actually 60.0 mph on 265/40R18, and revolutions per mile change from 766 to 765.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 235/45R18

Other comparisons with 265/40R18

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