15K is an overestimate for most bike tires. It might be accurate for ultra hard commuter ones, but my experience is more like 2k miles for a rear (at 700x25c, single racing bike) and 5k for the front. At that point, the rear is a pretty square shape, with thin tread in the center, and starts to have some interesting handling issues.
Wider, more supple tires might do better than that, the 26x2.2 rear on my tandem is 2k miles in, and probably has another 50% of its life left. Tandems are kind of noted for eating tires due to the loads on them.
