The spare on my WRX says on the sidewall something to the effect of "temporary use only, do not exceed 50 mph". Stick to these two bits of advice and your diffs should be fine. I can't believe your dealer replaced one tire, even on a 2wd car that is a no-no. Everyone with a driver's license *should* know that.

The reason tires have to match on most awd vehicles is because if the fronts are smaller around than than the backs (or vice versa), the front axles will be rotating faster than the rears. The speed differential between the axles increases with road speed. In a 5 speed WRX, this causes the center differentials viscous fluid to heat up, engaging the clutch packs inside, while the differential attempts to "equalize" the speed difference between the front and rear axles. This is how it is supposed to work, and, similar to a clutch, the differential is designed to do this for a very long time, but will eventually wear out, particularly at high speeds. Running tires of slightly different sizes will drastically shorten the life of a viscous center differential because the unit is constantly working at speeds much greater than what it was designed for...

Having said all that, I'm not sure how the DCCD works in your STi, but I'd guess similar principles apply to electronic diffs.

Cliff's notes: rotate your tires on a regular schedule and replace all 4 at the same time.

 

I caught a screw (literally somewhere on one of NY's fine parkways, and had to replace 1 b/stone potenza. I put the new one on the r rear, where the old one was, and have twice since rotated them. I also switched them from one side to the other: LF-RR, LR-RF, RF-LR, RR-LF. Can someone explain why this is not the recommended rotation, which is rears-front, front-rears, same side?

This happened at about 15K/miles, and the tread depths were very close (I do not take too many high speed drifts).