The price difference in tyres will be a bit, but probably not more then a couple hundred dollars. When you think about how long a set of tyres will last you its not a huge difference in price. I know I'd pay the little bit extra if I preffered the look of the bigger wheels... If you're forking out a fair bit of money for a brand new car, you may as well pay the little bit extra and have it just the way you would like it, in my opinion atleast.
Tirerack is the way to go....i've got 19's on my Mk5 GTI......quoted $600 a corner for Goodyear Eagle F1 Assym's (235 35 19). Had them shipped in 2 seperate shipments about 2 weeks apart (to avoid paying import duty) from Tirerack for $1300 for the set, plus $80 to be fitted locally....all up I saved about $1000. I agree they are also bery easy to deal with, just email them your request and they get a shipped quote to you in a couple of days.
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