Tried this app and it works great. Once you get a GPS signal the speedometer is fairly accurate. By keeping your efficiency score high over time you do indeed save on gas as well has help the environment. I also like the fact you can save your fill ups and it calculates your MPG for you. The heading indicator is ok and works majority of the time. Also has a handily general car info section which reminds you of your next scheduled maintenance, oil changes etc.