No on here has asked the question why slightly over-inflating those tires would increase gas mileage. It is most certainly because you have created a more "rounded" contact surface. Less surface contact equals less friction to cause rolling resistance. I would simply caution you that less surface contact also equals poorer handling.
That's not correct. Modern tire's are designed with metal ribs that force a tire to hold it's shape within reasonable pressure ranges. So the shape of the tire between 30 PSI and 50 PSI is not really changed when it's under the weight of the vehicle. It is a myth that higher pressure = more tire wear. This myth originates from pre-radial tire design, when indeed tires would be shaped like donuts.
It's more efficient because less energy is lost. Have you ever tried to ride a bike with a flat tire? A lot of energy is wasted trying to get force to the ground through a flat tire.
I have a lot of personal experience with this subject. On my last set of tires I ran them at about 50 PSI. Max rating was 44 PSI. Tread warranty on those tires was 80,000 miles. With regular balancing, alignments, and checking the pressure, I made them last the full warranty period before replacement. In my own data I discovered a 7% increase in fuel economy going from 30 PSI to 40 PSI.
What people have to realize is that the "Max Pressure" is not the true maximum pressure. There is a large safety margin put in place by the manufacturer. Risk management departments would never allow a manufacturer to state the actual maximum performance of a product. If you visit hypermiler forums you will encounter people who inflate their tires to 50, even 65 PSI without a single issue, despite the max ratings being 44 or 51 PSI. The tires can handle much more than their stated maximum, especially when these tires are for light commuter cars not driving under extreme circumstances.
Also realize that the tire pressure information for your car is calculated for maximum ride comfort, not efficiency or performance. A car with tires inflated to 30 PSI will ride more comfortably than an identical car with tires inflated to 45 PSI, without costing the car maker any money in suspension design. The reality is that a car will handle, stop, accelerate, and get better economy at higher tire pressures while sacrificing some ride and noise comfort.