As long as the OUTSIDE diameter of the wheels are the same you should be fine. If you didn't order anything strange tire size wise, then you shoul dbe fine. The only reason you would need to replace your spare would be if you installed a brake kit of some type and needed the larger wheel for clearance.
A8 Brakes require a 16" wheel, I believe porsche brakes require 17"+
My 18s go on today. The guy at the local garage also stated that if use the spare 15, put it on the rear. Otherwise you can ruin your transmission due to the differential. So, if I have to, in an emergency put the spare on the front, how long and at what speed would be advised driving on the spare?
Mike if the wheel / tire combo was sized properly the aspect ratio used on the 18s should give you the same overall OD as the 15s .. if thats the case there isn't a problem. If it wasn't and you have a flat on the front it means putting the spare onthe rear first then the 18 from the rear on the front..
No biggie .. think about it ,, if you have had good tires on your car for the last 5 years .. how many flats have you had? Not many if any.
No. The front differential on the FWD will tolerate slightly different wheel sizes (it just thins you're going around a corner). With the torsen, the extra rotration of a wheel causes the torsen diff to lock up. It shouldn't be a problem, but it's not desireable. I had a tire dealer install 1 wrong tire on my 95 quattro...it was noisier, but no actual problems surfaced in the 20 or so miles that it saw.