Tires: diameter of tires on Forester Subaru, subaru forester, tread depth

Question
Apparently my tires are wearing unevenly. The right side of my car's tire has  tred depth 7 something left on the tires. The left side has a depth tred of 4. While I could change both with the tred of 7, to the back, and buy two new tires, someone suggested that I purchase 4 new tires so that the diameter is the same. what do you think and why (in laymen's language)?

Thanks so much!
Moira

Answer
Moira,

It is my understanding that the owner's manual for Subaru Forester's says that the tires must be within 1/4" circumference of one another or there may be problems with the drivetrain.  Also 1/32nds of an inch difference in tread depth is about the limit.

There have been plenty of folks who know this to be true by first hand - and expensive - experience.

The problem is the center coupling and how much slip it has to give for tires with different diameters front to rear.

So it's 4 new ones or nothing.