225/40R17 vs 205/40R18

205/40R18 is 0.37 in (9.4 mm) taller than 225/40R17, a +1.54% diameter difference. With the speedometer set for 225/40R17, an indicated 60 mph is 60.9 mph on 205/40R18.

Is this a safe swap?

Diameter difference +1.54%: within the ±3% guideline

By overall diameter alone, 205/40R18 falls inside the ±3% range commonly used as a guideline when replacing 225/40R17. 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.

225/40R17 vs 205/40R18 differences
Measurement	225/40R17	205/40R18	Difference	% diff
Overall diameter	24.09 in (611.8 mm)	24.46 in (621.2 mm)	+0.37 in (+9.4 mm)	+1.54%
Sidewall height	3.54 in (90.0 mm)	3.23 in (82.0 mm)	−0.31 in (−8.0 mm)	−8.89%
Section width	8.86 in (225.0 mm)	8.07 in (205.0 mm)	−0.79 in (−20.0 mm)	−8.89%
Circumference	75.67 in (1,922.0 mm)	76.83 in (1,951.6 mm)	+1.16 in (+29.5 mm)	+1.54%
Revolutions per mile	837	825	−13	−1.51%

True speed with the speedometer calibrated for 225/40R17
Indicated	Actual on 225/40R17	Actual on 205/40R18	Difference
30 mph	30.0 mph	30.5 mph	+0.5 mph
60 mph	60.0 mph	60.9 mph	+0.9 mph
75 mph	75.0 mph	76.2 mph	+1.2 mph

Ride-height change: 205/40R18 raises the vehicle by 0.19 in (4.7 mm) at the axle (half the +9.4 mm diameter difference).

205/40R18 is 0.37 in taller than 225/40R17 and 20 mm narrower, a 1.54% difference in overall diameter.

With the speedometer calibrated for 225/40R17, an indicated 60 mph is actually 60.9 mph on 205/40R18, and revolutions per mile change from 837 to 825.

205/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 225/40R17.

Size pages

Other comparisons with 225/40R17

Other comparisons with 205/40R18

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