http://www.willtheyfit.com
-> check this out
Compared to the 8x20 225/45/18 ET30:
Compared to your existing wheel, this new wheel will have an inner rim which is 11.4mm closer to the suspension strut. The outer rim will poke out 1.3mm more than before.
Almost 12mm is a lot. Is it to much? I don't know, mine is only 6mm to the inside and I have plenty of space.
The site is full of info from people with different wheels 8.5x18 is a widely used setup on the site I have seen so far.
Good luck.