Originally Posted by whitetiger
it all depends on how worn the other tires are. take a tread depth measurement of the old tires, and then of the new. if the difference is less than .25", then you are ok.
No, No No. NOT .25" of tread depth variation.
The OLD Subaru requirement was 1/4" CIRCUMFERENCE difference. That's imperceptibly little variance, about 1/125", in tread depth. If the difference in depth is greater than 1/64th, it violates the old requirement. Most people believe, and are probably correct, that you are safe to +/- 1/32nd.
The NEW Subaru stance, requires all four tires to have the same degree of wear. There is no allowance for variation, make, model, or size. That stance is fairly definite.
To be safe, I'd replace all 4. It's costly, but the OEM tires on any Subaru leave a lot of the cars performance on the table.