Be careful about changing tyres on a quattro system. Ideally all tyres should have about equal wear on them so that their circumfrences are equal as well. Otherwise, the system can be damaged due to the difference in speeds at which the tyres are spinning. Check with your mechanic (or Audi) before replacing tyres. They will tell what the tolerances (acceptable differences in tread depth) are that will not cause damage to the drive system.
Good luck!
