One further comment I intended to add when I posted about hypermiling, but forgot: in the referenced article, overinflating tires is depicted to be extremely dangerous. In reality, overinflating by up to 20% is far less dangerous than the underinflated condition a huge percentage of people drive around on without even realizing it. In fact, a tire that is rated at 32 psi maximum is probably safer at 40 psi than it is at 32 psi, and is certainly safer than at 25 psi. The thing is, the lower the pressure in a tire, the more heat that tire builds up, and heat is the NUMBER ONE cause of tire failure. A tire that is inflated to 32 psi and then driven on the highway all day with a full load will probably heat up to a final pressure as high or higher than the same tire that is inflated to 40 psi.
Remember a few years ago when some Ford SUVs were crashing because of tire failure (and of course, the driver's lack of skill in dealing with it)? Ford successfully dumped the blame for that on Firestone (Ford has the money for more and better lawyers), but the real blame is just as much on Ford, because they specified 25 psi as the recommended tire pressure. Especially on a heavy vehicle, 25 psi is underinflation. So why did Ford specify 25 psi? So the vehicle would ride smoother.