235/45R17 vs 235/40R18

235/40R18 is 0.07 in (1.9 mm) taller than 235/45R17, a +0.30% diameter difference. With the speedometer set for 235/45R17, an indicated 60 mph is 60.2 mph on 235/40R18.

Is this a safe swap?

Diameter difference +0.30%: under 1%

By overall diameter alone, 235/40R18 is a near match for 235/45R17; 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.

235/45R17 vs 235/40R18 differences
Measurement	235/45R17	235/40R18	Difference	% diff
Overall diameter	25.33 in (643.3 mm)	25.40 in (645.2 mm)	+0.07 in (+1.9 mm)	+0.30%
Sidewall height	4.16 in (105.8 mm)	3.70 in (94.0 mm)	−0.46 in (−11.7 mm)	−11.11%
Section width	9.25 in (235.0 mm)	9.25 in (235.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	79.57 in (2,021.0 mm)	79.80 in (2,027.0 mm)	+0.24 in (+6.0 mm)	+0.30%
Revolutions per mile	796	794	−2	−0.29%

True speed with the speedometer calibrated for 235/45R17
Indicated	Actual on 235/45R17	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	75.2 mph	+0.2 mph

Ride-height change: 235/40R18 raises the vehicle by 0.04 in (1.0 mm) at the axle (half the +1.9 mm diameter difference).

235/40R18 is 0.07 in taller than 235/45R17 and the same width, a 0.30% difference in overall diameter.

With the speedometer calibrated for 235/45R17, an indicated 60 mph is actually 60.2 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 235/45R17.

Size pages

Other comparisons with 235/45R17

Other comparisons with 235/40R18

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