Tires: Singel tire replacement on an AWD vehicle, tire diameters, vw passat

Question
I drive a 2001.5 VW Passat 4motion AWD drive car.  Due to a series of events I have a single tire that needs to be replaced.  Today I was was not allowed to purchase a single tire due to the "manufacture requirement" that I have 4 matching tires.  The replacement tire I wanted was to be the same brand/model/size of the existing tire.  The retail store flatly refuse unless I bought 4 tires.  One tire has minimal tread and the other 3 are about 1/2 through their tread life.  I understand the requirement to have matching tires but I did not understand the need to have all 4 tires with matching tread wear.  Is this policy factual or was I given a line of BS.

Answer
Diane,

There have been plenty of reports of AWD vehicles having drivetrain failures due to differences in tire diameter.  Differences in tire diameters causes the components to bind up and eventually fail.  Obviously tires from different manufacturers or tires of different size would be different diameters, but even differences in tread depth seem to be enough to cause problems.

Some AWD systems are more sensitive to this than others and it is hard to get good reliable data as to which are which.

So the safe thing to do is to have 4 tires all the same.

However, if you feel confident that your vehicle is not one of those sensitive vehicles - and are willing to take the risk - then shop around.  You might find someone who is willing to mount an odd tire for you.