Just to note - 225 40 18 is a factory size for these cars and gives the most accurate speedo readings. If you compare the above images, you'll see that a 215 on your CH wheels brings the diameter much closer to the factory dimensions shown for the 225s on the VZ wheels.
That's another added benefit of going for 215s on the CH wheels.
The way it works is that the 215, 225 or 235 is the tyre's tread width in mm.
The next number, 40, 45, 35 etc is a percentage of the initial tread width number, giving you the height of the tyre wall, known as the 'aspect'. So the tyre wall of a 235 40 will be taller than a tyre wall of a 215 40.
40% of 235mm is more than 40% of 215mm.
It's only marginal, but becomes crucial in regards to maintaining accurate total diameter, and especially when you're trying to gain clearance and every mm counts.
Willtheyfit provides all that data, so it's a very handy tool.
Here's an image of a 215 40 18 tyre on an 8.5 wheel. You can see just how much clearance you can gain over a 235 40 18.