Buy a bicycle. (shvd, ya beat me to the punch).
Honestly, from looking at the mileage logs here on the sight, i don't see much disparity in the mileage from one to another.
Biggest thing I have found is keeping my speed down to around 65-70, and driving like a grandma, and while O do drive like an old geezer around town, i still drive 75-80 on the highway and my mileage shows it. If you go back and look at my logs, the best mileage I ever got was on the highway, and normally only when I was traveling with my father-in-law in tow where I was forced to keep the speed down.