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This app does what I want...to just track gas mileage. There's a lot of other bonus data for those who's interested such as average gas cost per day an such. I use it to track my services as well, such as oil change and tires change/rotation. I've been using this for a few years now (ever since I got the ipod touch 4). I'd say this app, (along with Byline for RSS) are the 2 third party apps thats been on my ipod touch for the longest time.
car tracker reminder If you own a car... gas usage mpg, and other car maintenance stuff = like tops2 said.
I've been using the free version for almost 3 years if that means anything. Bought the full version the other day and a single tap transferred all my data over. It's a great way to track all your mileage data. I also get reminders for things like oil changes which is great.