235/50R17 vs 265/40R18

265/40R18 is 0.09 in (2.4 mm) taller than 235/50R17, a +0.36% diameter difference. With the speedometer set for 235/50R17, an indicated 60 mph is 60.2 mph on 265/40R18.

Is this a safe swap?

Diameter difference +0.36%: under 1%

By overall diameter alone, 265/40R18 is a near match for 235/50R17; 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.

235/50R17 vs 265/40R18 differences
Measurement	235/50R17	265/40R18	Difference	% diff
Overall diameter	26.25 in (666.8 mm)	26.35 in (669.2 mm)	+0.09 in (+2.4 mm)	+0.36%
Sidewall height	4.63 in (117.5 mm)	4.17 in (106.0 mm)	−0.45 in (−11.5 mm)	−9.79%
Section width	9.25 in (235.0 mm)	10.43 in (265.0 mm)	+1.18 in (+30.0 mm)	+12.77%
Circumference	82.47 in (2,094.8 mm)	82.77 in (2,102.4 mm)	+0.30 in (+7.5 mm)	+0.36%
Revolutions per mile	768	765	−3	−0.36%

True speed with the speedometer calibrated for 235/50R17
Indicated	Actual on 235/50R17	Actual on 265/40R18	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: 265/40R18 raises the vehicle by 0.05 in (1.2 mm) at the axle (half the +2.4 mm diameter difference).

265/40R18 is 0.09 in taller than 235/50R17 and 30 mm wider, a 0.36% difference in overall diameter.

With the speedometer calibrated for 235/50R17, an indicated 60 mph is actually 60.2 mph on 265/40R18, and revolutions per mile change from 768 to 765.

265/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/50R17.

Size pages

Other comparisons with 235/50R17

Other comparisons with 265/40R18

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