I purchased a ScangaugeII on my first car and have used it on a number of different cars since. It does a reasonable job of giving an instant MPG but is not exactly dead on accurate as the old pencil and paper. You can adjust the accuracy at each fill up and it gets better over time.
I am sure there are other units similar to the scangaugeII that will give you the same information. They plug into the OBDII port
I just installed an android radio in my Smart and it came with the full blown version of Torque. I just a day or two ago set up all the questions it asks, but have only used not quite a half a tank of gas, so can't comment in how it does. It is loaded with alot of goodies. But a friend of my told me about an app called "Fuelly" and it's free in the google play store. He really likes it. So since you gave me a heads up on the Torque app I may try Fuelly to see how it does. It might be worth trying for your too. It's free so there's nothing to lose. I would really be interested in any of the Android apps you try for your Fortwo, so please post anything you tried and your opinion of the app. It would be most appeciated. Thanks.DCO