difference in RR
 

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 ?

I thought 5%.

From memory it's about 1%, which covers a new to worn tyre essentially.

when doctors disagree ....
lets put it another way
taking a new tread depth as 8mm , when you fit a new tyre should you change the other 3 tyres when they are at 7mm ?6mm? 5mm?
expensive hobby whatever

also if you fit a new pair at one end , how close do the pair at the other end have to be ?

You can get away with a 2mm tread depth variation across each axle and the average of the front and rear measurements can vary by 2mm as well. Only time that doesn't apply is with an electronically controlled diff when they reckon the rears should be as close as possible to the same tread

It's in elsawin as general guidance

sounds like advanced maths to me ,lol

to pass an MOT in france you must have the same brand and type of tyre on an axle with less than 5mm of tread depth difference down the centre ; so not as tight as a quattro!

could be a case for a front to back change on one side to keep to the 2mm limits as it works on averages , wouldn't have thought of that

That's a sensible thing from France.

I'v read into this before, and as much as I agree that all tyres should be of around the same depth along with the same make and model for Torsen Quattro, you have to consider that all 4 wheels are rarely spinning at the same speed unless you go to work on a straight road with no bends and reverse back home :ROFL:

I think Audi have a good set up on getting you to buy new tyres ;) and the honest answer is that it's probably not that finicky, but have I been brave enough to put it to the test? Nope :ROFL: so all my tyres are around the same depth and same make and model :D

I like it!!
Me, I just run them till they are worn and change one axle at a time, unless there is a load of expendable cash.
Seriously, being a skinflint pensioner, I'd run on second hand elastic bands if I could get away with it.

There can be quite large variations between makes and models of tyres, so taking into consideration the potential differences between 1.6mm and 7mm tread depths to the rolling radius, you can get outside tolerances fairly easily if you don't match tyre type.

The issue with electronic diffs is that they can get confused by tyres rotating at much different rates on the same axle but with no steering angle applied. Modern cars can be a bit too clever for their own good.....