Asammy - Thank you for asking this! I didn't even know I wondered, but reading the responses has been really informative.

Quote Originally Posted by RolliePollie View Post
This is what I go by too. Not flat in that they're not holding air, but flat as in more rectangular...like when you look at the tire from the side, there's almost an edge across the top because the road has worn the rubber sort of flat and wide.
Uh-oh, my rear one has looked like that for a while now... Maybe it's getting to be time. Then again, I haven't had any flats or issues with them so I might just wait a while longer.

Quote Originally Posted by 7rider View Post
I rotate them - maybe at the beginning of the season or if the rear is looking especially tired, so I can milk some more miles out of them.
Whoa, what a good idea! I would never have thought to do this. Crazy. I might just have to try switching the front & rear tires. I have Gatorskins and they're such a pain in the neck to get on and off, even after I've had to change a few flats with them, I usually avoid tire removal at all costs. But if I could squeeze another few months out of them... Hmmm...