I just started using this site, and it does many of the things my old app did, and things I wanted my new app to do. Now that I've found this, I'll not bother writing my new app. I'm quite excited (and disappointed for my own project).
I found a couple other stats to be useful per tank:
days it lasted
These let me acknowledge the tanks where I did a good job *reducing* my driving (or wear the over-driving as a badge of pride).
I also had graphs for fuel/day and price/gal just for good trivia -- how gas has changed price, old driving patterns, etc.
A lower fuel/day would be an enforceable behavior, and I often found myself referring back to the fuel price graph to show other people what I had collected.
I was also toying with the idea of accounting favorably for miles put on with more people in the car -- help those carpooling SUV drivers look better. "Sure I only do 10mpg, but I'm still saving the Earth by loading all 7 people into the car." Someone else here in the Tips had suggested increasing one's "People-Miles per gallon".
Really, guys, this is great work here on this site. Well done. I'm envious.
Yes, we're working on a new revamp of the entire site behind the scenes and one of the big features we are working on is custom additional stats, which will let you choose from a bunch of new stat options to add to car profile pages.