Replace all 4 tires at the same time?

[CJayArrgghh]

Hi all,

New C6 RS6 owner, picking it up next week. The dealership have installed two new tires on the front, but haven’t done the rears. I was under the impression that it’s best for an AWD to replace all 4 at once, is that the same with the Quattro? I’ve asked them to measure the tread depth on the tires that weren’t replaced to quantify the difference, assuming the front tires wear faster so there’s always going to be a level of difference in overall tire dimension over time. Is there any depth difference tolerance front to rear?

[IanH755]

Just because of a bad experience I had on another car (Mondeo of all things), I now ensure that all 4 tyres on any car I own are the same make/model, just to avoid grip differences causing a spin if cornering a bit quick, even in the dry

Outside of that, there's no warnings mentioned specifically in the owners manual about Front vs Rear having very slight differences due to wear. The only warnings are more concerned about having a too large overall rolling radius i.e. much wider tyres Rear vs Front etc, which isn't ideal for AWD i.e. something silly like 235/35-20 Front and 285/35-20 Rear etc would be very bad.

As a worst case, if you had the OEM size of 275/35-20 all around, and the fronts were brand new (8mm tread) and the rears were bald (1.6mm tread) then the rolling difference would be just 6.5mm, which is the same difference as having all 4 tyres being brand new (all 8mm) but with one set being 285/35-20 and the other being 275/35-20, which is a difference of less than 1% and therefore would be OK.

[Jim Haseltine]

A rule of thumb is that if the vehicle has 3 differentials (like a 'proper' non-Haldex quattro), then tyre wear/tread depth doesn't really matter. if there's a viscous coupling in the mix (like a Subaru or Volvo) then it matters a lot.
I've driven quattros for 30 years and never had a problem, not even after a winter where due to a slight lack of disposable income I commuted 50 miles a day with normal tyres on the front axle and chunky M&S on the back end. My V70 AWD stripped the angle gear in 18 months and I was lucky that the coupling didn't let go first.