ive been told that you can get 19's under the arches of a mk4 without any arch work.
the thing u have to look out for is the rolling radius changes.
click on this link, and scroll down to just passed half way, u should find a 'tyre size calculator' which will tell u how much difference there is between rolling radius' and how much your speedo will be out by.
http://www.carbibles.com/tyre_bible.htmli wouldnt of throught it would mess with the gearing, but then again, im no mk4 guru... but 18's arent uncommon, so it cant be that bad.
