Paul- I am on Fuelly, under name "Landsail". It's nice to be able to check the average of a lot of drivers and miles. What I don't like is the phone "app". It's not really an app, just a downloaded form. I couldn't even move it enough to enter the data, and it can't be uploaded anyway. So I'm still writing data on receipt, and enter it later at website. This caused me to mess up once, I entered odo mileage when I got home, instead of mileage at fillup. It's noted in note section.
I'd like to use a site that you can enter data from phone, but most have to much stuff for business miles, maintenance, etc., which I don't need. So Fuelly works well to get a real world average. So far after 2k+ miles I am averaging 54+ miles. Not to shabby.