235/40R17 vs 205/40R18

205/40R18 is 0.06 in (1.4 mm) taller than 235/40R17, a +0.23% diameter difference. With the speedometer set for 235/40R17, an indicated 60 mph is 60.1 mph on 205/40R18.

Is this a safe swap?

Diameter difference +0.23%: under 1%

By overall diameter alone, 205/40R18 is a near match for 235/40R17; 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.

235/40R17 vs 205/40R18 differences
Measurement	235/40R17	205/40R18	Difference	% diff
Overall diameter	24.40 in (619.8 mm)	24.46 in (621.2 mm)	+0.06 in (+1.4 mm)	+0.23%
Sidewall height	3.70 in (94.0 mm)	3.23 in (82.0 mm)	−0.47 in (−12.0 mm)	−12.77%
Section width	9.25 in (235.0 mm)	8.07 in (205.0 mm)	−1.18 in (−30.0 mm)	−12.77%
Circumference	76.66 in (1,947.2 mm)	76.83 in (1,951.6 mm)	+0.17 in (+4.4 mm)	+0.23%
Revolutions per mile	827	825	−2	−0.23%

True speed with the speedometer calibrated for 235/40R17
Indicated	Actual on 235/40R17	Actual on 205/40R18	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: 205/40R18 raises the vehicle by 0.03 in (0.7 mm) at the axle (half the +1.4 mm diameter difference).

205/40R18 is 0.06 in taller than 235/40R17 and 30 mm narrower, a 0.23% difference in overall diameter.

With the speedometer calibrated for 235/40R17, an indicated 60 mph is actually 60.1 mph on 205/40R18, and revolutions per mile change from 827 to 825.

205/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 235/40R17.

Size pages

Other comparisons with 235/40R17

Other comparisons with 205/40R18

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