Tom -- another reason why the difference in performance with the 315s in the rear... vs a square setup (275x4)... could be that the 315s are a touch taller in diameter.. thus shifting a bit of weight towards the front of the car (changing the balance that you're used to)... and actually changing the alignment slightly by reducing the positive caster (having taller rear tires lifting the rear).. from whatever/whenever you had it aligned at. When you had the alignment done, did it have a square tire setup? (all 4 tires the same size/diameter?)