i believe that one of the reasons for increasing the inflation pressure for sustained high-speed driving is to reduce the sidewall flex at those high speeds. Sidewall flex creates internal friction, which raises a tire's temperature.

the same is recommended if you go autocrossing, so you don't run your tires off your rims.

different cars have different suspension design characteristics, and the tire itself is part of the suspension. Inflation pressure determines the amount of road shock the tire absorbs, and the shape and behavior of the contact patch, among others.

i've read somewhere that it takes a few hours for a tire to return to normal temperature, since rubber is such a good insulator. Also, if you cannot inflate your tire to the recommended pressure before driving off, be sure to do it within about 1-2kms (and drive slowly) or else your tire will have heated up considerably and you will get readings that are way off. Even then, you should provide some margin.

also, invest in the best tire gauge you can get your hands on.

my Can$0.02