What about keeping track of vehicle maintenance records also? Something like oil changes, tire rotation, etc. This could provide a one stop shop for all maintenance records, millage, and so on.
This would also provide vital details to someone else if you were to sell your car. It would be as easy as logging on to Fuelly and printing all the records out, or just giving the buyer the URL for your site.
This is the idea behind vehicle notes. Click 'Add a note' from the menu to the left to try it out. I add oil changes via notes, and you can see how others are using notes here: Recent Activity showing New Notes. Just click their car name and scroll down to their recent fuel-ups to see how notes fit in with fuel-ups.
Another cool thing that I have noticed, believe it or not, is that when I needed to replace my tires my mileage went down with the ones that I chose. Maybe, that's not a cool thing. I guess it was cool that I was able to notice it since I was recording everything. I didn't expect the brand of tires to affect my mileage the way it did. That would let everyone see how a particular brand of tires can change what you're getting on gas mileage.