I drive like a maniac. 75 or higher on I-5. When I filled up for gas last Saturday some quick math (gallons/miles) showed me my 97 Toyota Carolla got 31.76 miles per gallon.
I decided to try a little experiment. I would only drive the speed limit, or under. It was difficult, but I persevered and WOW does it make a difference. I noticed something was going on when I reached 360 miles on my 11 gallon tank and the gas light still hadn't come on. I ended up getting to 400 miles for the first time, and ended up with an astonishing 37.9 mpg! That's an increase of 6.13 mpg or nearly 20%
It's not rocket science, just drive the speed limit.