I've found the info that I recalled seeing (from
http://www.pistonheads.com/GASSING/topi ... alf%20worn):
"Secondly, and this is more important, simply because it is more likely to occur, and it regards the 'Load Index' of the tyre. When the B6 S4 was released in 2003, it intially came with 'standard' load tyres with a load index of 91 (so the sidewall marking would be "235/40 ZR18 91Y"). However, these were borderline to say the least, and would cause excess wear on the outer shoulders, and could also cause the 'sawtooth' effect on the standard ContiSportContact2s. From model year 2004, the standard fit tyres on the S4 were improved to the "eXtra Load" load index rating of 95 (full sidewall marking "235/40 ZR18 95Y XL") - and not only did this improve car stability under very hard braking and hard cornering, it also noticeably evened out the tyre wear. It was recommended that these 95 XL tyres were retro fitted to the original B6 S4s come replacement time.
If you do have the lesser 91 tyres, then you have only two options - either get the tyres replaced with 95Y XL flavour (at someone elses expense - if it was an Audi dealer or other dealer who incorrectly fitted them, then they should be liable for the costs). If that fails, then you really need to increase the tyre pressures to cope. As a general 'rule of thumb', for every one point in load index variation, you need to adjust the pressure by 1½-2psi - so in your instance, increase the pressures by 6-8psi from that on the door pillar sticker."
