[thstone]

Alignment and/or air pressure.

Normally, the standard rear alignment will wear the inner 2/3's of the tire. The outer 1/3 barely sees any wear until you push the car hard in the corners and the camber changes to straighten up the tire so the outer 1/3 sees action.

If the alignment is off, then the car could be running mostly on the center 1/3 of the rear tire rather than on the inner 1/3 + center 1/3. This would tend to double the wear on the center 1/3.

If the air pressure is high, then the rears will wear more towards the middle as the air pressue will reduce pressure on the inner 1/3 of the tire and have the same effect as poor alignment.

Have a good alignment done and consider reducing the rear air pressure a few pounds.

Also, check the passenger rear for anything bent or worn like a wheel hub or suspension component.