The harder you ride, the faster your tires wear, especially braking and climbing if you put out a lot of power. The contact patch on 25s will be narrower than on wider tires, so yes, the rear will show a center flat earlier, etc. There's less rubber, so wear is concentrated in a smaller area. That said, around here I never wear my tires out. They always get cut up before that happens.
I always have at least one set of new tires in the spares tote.
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