I go through 2 rear tires for every front one. I base it on # of flats and cuts in the tires. I still get more rear flats. I could probably go 3 rear tires for every front one. I wonder if it's not just wear, but that I miss more glass with the front because I can see it, but then still hit it with the back?

There's a lot of glass on the roads I commute on. I've only had one flat ever in my town, but lots on the main road to work. If I just rode here, I'd probably get way more miles out of my tires.