Based on my experience 13k miles is within the "normal" range of the stock rear tire wear rate. I'm currently running PSS tires on my '12 and '09 M3s, and based on current tread depth and miles traveled, I'm estimating that I'll get ~20k miles out of both sets of PSS tires. My stock 18" PS2s lasted ~14k miles of street-only driving - this pretty much agrees with Michelin's statement that the PSS wear rate is much better than the PS2 wear rate.
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