Hi there John. Welcome to the forum.
These 18" TT alloys have been discussed a few times. As have 18" rims in general.
If you do a search for TT in the Wheels&Tyres subforum (this one) and check the box for 'Archived posts' you will get a load of posts related to the TT and large rims.
Here's one I've lifted out for you:
http://www.a2forum.net/forum/topic.asp?ARCHIVE=true&TOPIC_ID=2847&SearchTerms=tt
I trust jon.od in that post since I know that he seriously looked at large wheels. And he settled for 17" because 18" is just a bit over the top and it just takes too much effort, aggravation and money...for very little extra.
So not a definite 'no' here (you can always use spacers and/or modify the arches
). But read some of the threads you find in the archive and decide for yourself.
HTH,
Edo
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(2003 Silver 1.4 Tdi Exclusive(SE), 95bhp/235Nm chiptuned by ABT, black interior, sport seats, heated mirrors, TomTom3 PDA Satnav, JVC KD-SH909RB cd-receiver, Infinity Kappa 63.1 + Vifa on-dash tweeters, 12" Subwoofer on 300w Amp)