I just go with what Audi say on the sticker in the door shut. As I generally only have just me and my wife in the car, I went with the pressures for up to 3 people and 1 suitcase (?). 42 at the front, 36 at the rear in my 225/40x18's. It feels fine, I'm not getting any unusual wear and I get around 60mpg on my 10 mile drive at 60mph on the M1.
Being a diesel, I always assumed the pressures were so high because of the weight of the diesel lump. Although whether that's still true for modern diesels I'm not sure.
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Surely running much more than a few psi below the amount on the sticker will affect the mpg?
Michellin reckon that 7psi under the recommended amount is dangerously underinflated. (I think they may be exagerating slightly though).
Check your tyre pressures at least once a month | Care Guide | | Learn and Share | MICHELIN
The AA also say that 8psi underinflated will cost you 10% in mpg. About 5 or 6 mpg in a diesel. Again, this is probably debatable though.
I wish I knew more about tyres, there's so much conflicting information. It sounds like most people on here are running less than what Audi recommend and not experiencing any issues.