I was getting pretty consistant 17 all around and 20 highway (on 267 mile trip to Maine).
Then I found something called a hydrogen cracking system on the web, bought the materials and made one and installed it. My mileage went up 18.5 % (to 20.1 all around and 23.7 on the same trip to Maine). I used this system for a few months, then the 45K mile service came due, so I took it off (so they wouldn't give me attitude about possible warranty voiding issues) and I have not bothered to put it back on again. It got to be a pain it the butt adding 6 ounces of gas to a "bubbler" every two days. There isn't enough room under the hood to build a bigger one.
So I am back to 17 / 20 using premium fuel.