AWD and tire circumference difference

[MechE00]

Regarding running a staggered setup on an all wheel drive 3 series, does anyone have experience, or have an authoritative source they could cite or direct me to for what kind of percentage difference in circumference is too big of a difference?

For instance, the diameter difference between a new pair of tires on the front and a worn pair of tires on the rear could easily be greater than the difference between running a different series of tire... has anyone had problems from replacing only the front two or back two tires of their X BMW?

Many thanks...

[MechE00]

Hmm.. after some further reading, it seems the reason to avoid the different tires sizes is to avoid excess spinning of the planetary gearset in the center diff, and the extra heat that would cause... So as long as you do not end up with an overheating problem with it, you should be alright-- though it would be prudent to keep up with scheduled fluid change intervals and check the quality of the outgoing fluids...