Originally Posted by dmanaenk
Subaru's official recommendation on circumference difference (1/4") comes down to about 3/64nds of thread depth difference. There is probably some safety margin in there somewhere, so depending on what exactly is 90% new (any my how much did the seller lie) - you might be okay. Basically if new are @12/32nds, and used one is at 11/32nds I'd say you're okay.
Subaru retracted that "official" statement several years ago. The new, official, statement is what I posted above.
Any difference in degree of wear has the potential for permanent drivetrain damage. All 4 tires need to be the same.