I've been using Gas Cubby for over a year now. You just log your odometer and fuel fillups and it calculates everything else. It also allows you to put service stuff in and sync to their servers in case you have multiple people driving the cars with separate phones. You can keep track of multiple cars too.
I actually use a website called Fuelly.com. It will allow you to add multiple cars, post a badge on websites (if you want) that lists your car, a picture of it, and the gas mileage you are getting. You can now update your information using SMS text. It has all kinds of statistics to look at after you get a few fuel-ups added in.
If you have your heart set on using an actual app, you can log to the website with your phone and save the icon of the website to your phone so that it actually looks like a program and will act like one too. Here is what the badge looks like.