I have been using fuelly.com for a little while now. I wanted an android app that I could use to submit fuel-ups.
I've looked at the fuelly app, but i often don't have a data connection, which renders it useless.
I've looked at aCar, not what I'm looking for. That is more complicated than I want, plus to get the data on the app and online, you would have to constantly export/import the data between the systems.
There is an app, but it's basically just a text (sms) formatter (using the sms text system that fuelly.com has in place). I think it's easy to use. Just type in the mileage, price, and gallons (options exist for notes, city %, and using multiple vehicles), then hit the send button.
Of course this app isn't supported by the fuelly.com developers at all. It's free and there are no ads. I like it. Hope others can find use out of it.