Originally Posted by RonMaiden
Overinflating the tires will improve MPGss but wearing them out quicker will cost you more and with a smaller contect patch you also have less grip in corners. Don't overinflate.
Yes and no. Manufacturers often set tire pressure lower than optimal to improve ride comfort, since tires are also an integral part of the suspension system. A good way to actually check for overinflation after setting your desired pressure is to chalk a line across your tires and roll your vehicle one or two revolutions in a flat parking lot. The chalk will leave a (witness) mark on the asphalt -- if a tire is overinflated, the line will not span the full width of the tread. In that case, drop your pressure and try again until the mark looks good. By using this technique you can see how far the tire pressure can be raised without overinflating.
It's also interesting to note that pressure can fluctuate more than one might imagine. My Jeep has a direct readout TPMS system so it is possible to monitor pressure continuously in each tire. Driving from San Diego to Phoenix for example, it can vary up to 7 PSI gain due to
1. internal friction (1 to 3 psi)
2. air/pavement temperature increases (up to 4 psi)
3. solar contact (1 or 2 psi)
So "cold inflation" specs would ideally be set before driving and not in the sun. If it is necessary to air the tires at the gas station for instance, you should add 2 or 3 psi to your target pressure -- you can bleed any excess off later after the tires have cooled off.
I typically set about 38 PSI cold. This should help the mileage fractionally without overinflation while insuring the pressure doesn't rise above 45 MAX after potential gains, yet going no lower than recommended after a month of natural bleed off or cold spell.