Originally Posted by wstar

You'll hear some good rough guidelines, but the bottom line is you'll get the best exact reference by looking at your own tires after a run. I'm still new at this as well, but my understanding of the very basics is (someone please correct me if I've gotten something backwards):

1) Look at the edge of the wear pattern after a run. If it looks like on corners you're wearing on part of the tire that's past the edge (there's a marker on the tire for where the true edge is), you need to increase pressure to prevent the tire from rolling over so much. Similarly, if you seem to be not using enough of the edge, you can drop pressure a bit to gain more grip.

2) Check tire temperatures across the width (e.g. near each edge, and in the center). They make proper tire pyrometers for this that use a thumbtack-type sensor you stick into the rubber, but you can also use a regular infrared thermometer to roughly do the same thing. You want reasonably even temperatures across the surface. If the edges aren't as hot as the middle, drop pressure to flatten the tire out a bit more.