Tires: tire wear on 4WD vehicle, tire diameters, tire store

Question
Hi Barry,

My wife has a 4WD Audi and recently picked up a nail in the side wall of one tire. At the tire store, they told her since it couldn't be repaired, she would need to replace all 4 tires   since it is a 4WD. They explained that not doing so would ruin the transmission...which I very much doubt. What do you think? By the way, the tires have about 60% remaining tread life. Thank you very much!

Answer
Woody,

I am afraid the tire store is right.

AWD and full time 4WD have a center coupling and most of these require the tire diameters of the front to be the same as the rear (or within a reasonable tolerance).  If the tire diameters are different, then the center coupling has to slip - and it can tolerate some slippage - but too much slippage will cause it to fail.  There is plenty of anecdotal evidence on the Internet to confirm that this theory occurs in actual practice.