One thing you may notice, as A LOT of people have with any of the tuners out there right now, is that with their "economy" or "mpg" tunes you rarely gain any mpg. In fact most people (including myself) saw a decrease in fuel economy running the econo-tunes. With my Superchips, running the 87 ecconomy tune I saw a drop of 2mpg. I was getting 8-9mpg running that tune. Went to the 91 performance tune it jumped to 12-13mpg. Swapped the Superchips a few years later and went with the Diablo Sport Predator (DSP) tuner as I was wanting to die into my engine and start adding a few bigger mods. From the Superchips 91 performance tune to the Diablo's 91 performance tune, my mpg's went up to the 14-15mpg mark. Again a good jump. Tried the 87 econo tune and it dropped back down 12-13mpg consistantly so I went back to the 91. Now with a boat load more mods and a custom tune I sit with a solid 16-18mpg.
Now I also gavew the tunes each time I tried them at least a month and a few fill ups and did all my calcs with a pen and paper. Im not saying that one tuner is better than the other either, just what I noticed at the time when I was swapping things around a few years ago. The way I look at it though, 91 octane (must be ran for the 91 performance tunes at the very minimal) is only $0.20 higher in cost. So on a full tank of gas, thats only an extra $4.80. Going to a fast food place for lunch costs more than that. Also if it is saving you an extra 2-3mpg thats around $12-$15 you are saving per fill up.