205/45R17 vs 205/40R18

205/40R18 is 0.19 in (4.9 mm) taller than 205/45R17, a +0.80% diameter difference. With the speedometer set for 205/45R17, an indicated 60 mph is 60.5 mph on 205/40R18.

Is this a safe swap?

Diameter difference +0.80%: under 1%

By overall diameter alone, 205/40R18 is a near match for 205/45R17; 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.

205/45R17 vs 205/40R18 differences
Measurement	205/45R17	205/40R18	Difference	% diff
Overall diameter	24.26 in (616.3 mm)	24.46 in (621.2 mm)	+0.19 in (+4.9 mm)	+0.80%
Sidewall height	3.63 in (92.3 mm)	3.23 in (82.0 mm)	−0.40 in (−10.2 mm)	−11.11%
Section width	8.07 in (205.0 mm)	8.07 in (205.0 mm)	0.00 in (0.0 mm)	0.00%
Circumference	76.23 in (1,936.2 mm)	76.83 in (1,951.6 mm)	+0.61 in (+15.4 mm)	+0.80%
Revolutions per mile	831	825	−7	−0.79%

True speed with the speedometer calibrated for 205/45R17
Indicated	Actual on 205/45R17	Actual on 205/40R18	Difference
30 mph	30.0 mph	30.2 mph	+0.2 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 205/40R18 raises the vehicle by 0.10 in (2.5 mm) at the axle (half the +4.9 mm diameter difference).

205/40R18 is 0.19 in taller than 205/45R17 and the same width, a 0.80% difference in overall diameter.

With the speedometer calibrated for 205/45R17, an indicated 60 mph is actually 60.5 mph on 205/40R18, and revolutions per mile change from 831 to 825.

205/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 205/45R17.

Size pages

Other comparisons with 205/45R17

Other comparisons with 205/40R18

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