Transfer Case Tire Tread relationship
 

Recently purchased a used A8L, 2007, 77,000 miles. Tires appeared in reasonably good shape. Had a blow out, and went to purchase 1 tire. Was told Audi recommends replacing at minimum both tires on the same axle to avoid tire tread depth discrepancy which can or will cause the transfer case to fail. Tread depth must not exceed 4/32" (an 1/8th?). Tire dealer would not sell me 1 tire for fear of having to replace my transfer case?
I'm new to Audi and 4 wheel drive. Does this sound right? Because, at face value, it sound like BS to me!

It's not BS. The Torsen center diff uses the difference in wheel speeds across the axles to control how much torque goes to which axle. Sadly, the diff cannot tell the difference between wheel slip and a set of tires that are worn to different tread depths, or are of different sizes.

It's not about matching tires on the same axle to each other (as the front and rear diffs are open), it's more about the rotational speeds of the driveshafts from the center diff to each drive axle that matter. This is influenced by the overall rolling diameter of a pair of tires, comparing the front to the rear.

Now, what the bigger question becomes is how much discrepancy in rolling diameter of each tire becomes problematic on a dry road at normal speeds. Obviously, you can't run different size tires, and even staggered setups (which are an absolute move in the wrong direction anyway on an AWD car) can potentially be problematic (285/30-18 is just about the same rolling diameter as 225/45-18 but not quite).

In the real world on any AWD car, I would never try running anything but 4 matching tires, obviously of the same size. Overall diameters vary in the same size from manufacture to manufacturer and model to model. Now, if you just bought a set of tires 1000 miles ago and stick a huge nail through one of them, go ahead and replace the one tire. However, if any of the other 3 show measurable wear from new, I'd be buying a new set of tires.

Thanks for the feedback, BMWM750. Definitely a lesson learned today.

Meanwhile, on the practical side, you can...
 

often buy a match to a used tire on eBay. Obviously you want the same brand and similar wear. If it's a current model tire, TireRack will also shave a tire down if you want to go that way.

The handbook has all this information, and is worth a read.
I would also try and get a matching used one off ebay, as a different tyre may not only have deeper tread but different deflection characteristics too.
While it may not cause a problem, if you did ever have drivetrain trouble, even a slight whine, you'd be forever kicking yourself and wondering if this caused it.

Guys good info. So the "bottom line" not a good idea to miss match tire and tire size?

Absolutely not. AWD drivelines are a lot more sensitive to this than most people think.

Consult the Tirerack website. They offer a service where they will shave a new tire to match your current tread depth.