I think why we flat based on a gash is much more complicated than a puncture. I usally change my tires when they are noticeably not rounded, has a visible large gash or explode (eek). I have changed some I thought looked a little sad and found gashes but never flatted.

On a side note, I don't get more than 3,000 miles because I have the depth perception of a drunken armadillo and run over all kinds of stuff. DH said after the cut tire in Taos I can only ride Gatorskins or Armadillos from now on. If I were fast I might care but whats a little extra weight on a turtle? I will work on dropping pounds not grams from the engine (me).