With apps such as MPG, Mileage Tracker, Miley, and more, plus a bevy of web apps here and more coming, I have a question. So the apps have me input the fuel amount purchased, fuel price and odometer/tripometer reading. My question is, how do the apps know how much gas is left in my fuel tank, or is that not necessary. Are they assuming that every time I get gas I am on completely empty? How do I work this out? I'm sorry if I am not phrasing my question right. Thanks!