Alloys

Standard TT wheels come in various sizes from 16"-18".
Offsets are ET31, ET32, ET33 for 16(5spoke),17(5/6spoke),18(RS4) inch wheels

The TT has a hub dimension of
5/100pcd
(Pitch Circle Diameter)

The main tyre dimension is 225/45/17 for 17 inch alloys

Any substitution should be based around these
dimensions to keep within the cars specifications,
these can be compared with a tyre size calculator.

UPDATE2: Tried some more:- 19" RS6's

UPDATE:- I just bought these..........:o)
Thanks to Steve_TT for the heads up, WheelbaseUK on ebay supplied me these lovely Diamond wheels.

The other thing to remember is different tyres have different profiles, even though they are the same size.

Although it looks minor the difference in the tyre shoulder profile of these two tyres is visible as is the additional rim protection, this difference is enough to change my setup to being noise free to rubbing on most heavy dips.

The wheel arches are cavernous and can take a variety of offsets and wheels up to 19".

It is possible to fit a wide range of wheels however consider your warranty if you do so.

Front 18x8.5 with 225/40x18 ET 29
Rear 18x10 with 255/35x18 ET24
has been fitted to give an exaggerated rear end look to some TT's
and these tyre sizes fit into the standard rolling radius specifications.

My own wheels are Allessio Vuelta's 18x8 ET35 with 235/40/18
 which is just marginally bigger on the rolling radius but within tolerances IMO.

Available from Motionstyling

Alessio's on Black!