205/50R17 vs 225/40R18

225/40R18 is 0.02 in (0.4 mm) taller than 205/50R17, a +0.06% diameter difference. With the speedometer set for 205/50R17, an indicated 60 mph is 60.0 mph on 225/40R18.

Is this a safe swap?

Diameter difference +0.06%: under 1%

By overall diameter alone, 225/40R18 is a near match for 205/50R17; 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.

205/50R17 vs 225/40R18 differences
Measurement	205/50R17	225/40R18	Difference	% diff
Overall diameter	25.07 in (636.8 mm)	25.09 in (637.2 mm)	+0.02 in (+0.4 mm)	+0.06%
Sidewall height	4.04 in (102.5 mm)	3.54 in (90.0 mm)	−0.49 in (−12.5 mm)	−12.20%
Section width	8.07 in (205.0 mm)	8.86 in (225.0 mm)	+0.79 in (+20.0 mm)	+9.76%
Circumference	78.76 in (2,000.6 mm)	78.81 in (2,001.8 mm)	+0.05 in (+1.3 mm)	+0.06%
Revolutions per mile	804	804	−1	−0.06%

True speed with the speedometer calibrated for 205/50R17
Indicated	Actual on 205/50R17	Actual on 225/40R18
30 mph	30.0 mph	30.0 mph
60 mph	60.0 mph	60.0 mph
75 mph	75.0 mph	75.0 mph

Ride-height change: 225/40R18 raises the vehicle by 0.01 in (0.2 mm) at the axle (half the +0.4 mm diameter difference).

225/40R18 is 0.02 in taller than 205/50R17 and 20 mm wider, a 0.06% difference in overall diameter.

With the speedometer calibrated for 205/50R17, an indicated 60 mph is actually 60.0 mph on 225/40R18, and revolutions per mile change from 804 to 804.

225/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 205/50R17.

Size pages

Other comparisons with 205/50R17

Other comparisons with 225/40R18

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