Stock Legacy is (I think) a 15X6 wheel, 5X100 bolt circle, 35ish mm offset. The 16 inch wheel is an upgrade (used to be, anyway), probably a 16X6.5, everything else pretty much the same. The center "hub" diameter varies as well; don't know what the Subaru number is, but ideally the hub opening will just fit over the shoulder of the hub. It can be larger w/o any real issues, but obviously not smaller. The 215/60 is gonna be probably an inch taller than the 205/55 as well; your speedo will be off 5-6%. (Not that the Subaru speedo is all that accurate to begin with, but you will see a definite change.)
Ummmm, what's a "Mercury Sable", anyway ;-)
Seriously, tho, the best (and really the _only_) way to be sure is to try 'em on. Feel for clearance between the strut and tire, visually estimate clearance between the outside of the tire and the inner lip of the wheel well. Try both front and back. If it feels real close (
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