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Do you have the staggered wheel setup (different width in the back than the front)? If so, then you can only rotate them side to side, and on a RWD car the rears will wear out faster than the fronts. If they're directional tires (check the sidewall for an arrow indicating the direction of rotation), then you can't rotate them side to side. If you don't rotate them, the right rear will normally wear out faster than the left.
What model is the tire? 15k still seems low unless it's a high performance tire or you drive very aggressively.
Are they wearing evenly, and have you had your alignment checked recently? Uneven wear could be caused by a bad alignment.