4 Tires or 2?
 

Need new tires on the front so went to the tire dealer and was going to get two new ones. He told me that since it was all wheel drive, I would have to get all 4 tires at once since they all need to be the same size. Made some sense, but they are not the same size now. The fronts are worn out and the backs still have some tread left. If I get two new ones, the new ones will have more tread than the ones I leave on, which is the same circumstance I have now. Is he blowing smoke or do I need to replace all 4? Thanks

It is good practice to replace all four.

It is true that the front to rear difference can cause problems with the transfer case. Also, different tires brands can have different rolling diameters in the same size, so if you do mix them it is safer to do so if they are the same brand and model, as well as being the same nominal size.

BMW's spec for allowable tolerance in rolling diameter is posted here, from memory it is a few revs per mile difference. If you use the same brand and size tire, you are safe to about 50% tread difference in wear as a cutoff. So, if your old tires have 60% tread remaining, you can buy two. If they have 30% remaining, you should buy four.