There needs to be a "It didn't quite get to full." option.
One thing that is obvious after my migration from My Mile Marker (which I liked, but wasn't all that social) is the "It didn't quite get to full." option. You can't really count gassing up to 3/4 of a tank in your averages and get accurate numbers.
Also, "I didn't record my last fill up." would be nice to have as well. I'm mostly worried about accurate averages, y'know?
Yes, I came from mymilemarker.com as well, and the first thing I noticed here was there wasn't a way to put that you missed a fillup. If for some reason your last fillup didn't get recorded, it should be noted somewhere so it doesn't mess up the numbers.