Hypermiling: Informal player

So, a coworker gave me some numbers for a parallel experiment that he did (and is continuing). He drives a lot farther than me every day and has a smaller tank so he's on a faster pace than I am. The following are the results of his initial experiment (in his Kia Rio).

In one tank of gas with normal driving, he got 27.2 mpg. Then he checked the air pressure in his tires and found that three were at 35psi but one was at 22psi. The recommended pressure is 35psi and the maximum is 44psi. So, he filled them all up to 42psi and drove for another tank of gas.

Results: 32.3 mpg, an improvement of 5.1 mpg just by overinflating his tires (and fixing the one that was underinflated previously). That's pretty impressive, now I'm encouraged that I'll see some real results in my experiment.