Hi... I'm new to Fuelly but, have been a "car person" since I started driving in 1977. The online fuel logger here seems very good. In the past, my favorite "app" was a home brew Excel spreadsheet. I don't bother with "smartphones" anymore but if went that route, I'd make sure there's a way to capture the data in CSV format so when the phone itself or the service behind the "app" fades to oblivion, you can still keep your data.
Don't Let The Sound Of Your Own Wheels Drive You Crazy