This is interesting:

http://www.popularmechanics.com/blog...s/4199963.html

"To invoke the MythBusters, we’d now consider the myth of increased fuel economy from over-inflated tires officially busted. So why didn’t the harder tires with less drag result in better mileage? Well, you’ve got to figure that, at 70 mph or so, the aerodynamic drag of the car’s body is probably outweighing the small improvements the tires gave us."