I found this place to be very useful recently, especially when checking out tyres and wheels etc.

http://www.carbibles.com/ 

there is a section where you can put in what your current or cars tyre specs are...and change the rim size etc etc unti you get something ur happy with that should affect to heavy on the cars performance. (i'm sure i havent expained it correctly)

Anyway...it was super helpful for someone like me when i had to work out what the effect was when going from a 14 inch rim to a 15 inch..and what was the best pofile tyre to run with without effecting the cars performance or throwing out the speedo.

I didn't have to do the calculations...it did it for me.

It puts it all into laymens terms too.

anyway hope u have fun with this stuff too.

PS: it not VW specific...

The Tire size calculator I use is :

http://www.miata.net/garage/tirecalc.html 

its an excellent site...

shows all sizes in Metric or Imperial

shows the difference between two tyres...

I haven't looked at the bible sight...
maybe on Sunday... lol

LEE