the 2005 GTi had 17x7 wheels as an option so you are allowd to use a 16x6.5 or a 17x7 that has an offset within .25" of the stock offset.
The GTi has plenty of acceleration without help for autocross purposess. What you want is the widest tire you can fit on one of those size rims/under your fenders, which is 225 mabey 235. The diameter of the rim dosen't matter much just try to find a set of lighter weight rims(stock rims are usually ~22lbs+). If you are going to drive these tires on the street also you will want both a "street tire" and close to stock diameter(ie. 225 45 17) if you are going to change tires at the events you can run a DOT R-comp tire (usually has a treadwear less than 100) and a smaller tire(ie 225 40 17)
that being said if it is your first time buying autocross specific tires I would recomend street tires as most local regions have a street tire stock class or street tire handicap for stock classes. Most people run and like the Hankook ventus z212 rs2 and the Falken azenis rt615.
http://www.edgeracing.com has lots of lighter wheels and often has sales on their wheels and/or tires
I hope this helps and good luck
Modified by meangreen18T at 5:25 AM 3-23-2007
Modified by meangreen18T at 5:26 AM 3-23-2007