difference in RR

[oldnick]

having been glancing through some french Audi websites I notice that a number of people have been told by their audi dealers that if they have to change a tyre on a quattro and the others are more than slightly worn they must fit a complete set .... clearly due to the difference in RR

but I presume there must be a tolerance within which the system doesn't get damaged ....question is , what is it ?