This is a great open source MPG instrument for your car. It’s based on the Arduino micro-controller along with a few other sensors. I really like all these open-source projects that are springing up all over the place. Amazing!
MPGuino is part of the OpenGauge instrumentation effort, which was created to make available efficiency instrumentation of all sorts. MPGuino specifically is a Miles Per Gallon gauge for fuel injected vehicles that is based on the arduino platform with some extra circuitry for monitoring the fuel injectors and the speed sensor in the vehicle.
More about MPGuino: Open source instrumentation