Current Mileage LED for cars

I want people to know it's possible to get 43 mpg in the city. My Jetta TDI can do it.
It would be cool to have the thing calculate my current mileage and put that number in real time on an LED readout across my back window. Good for the Prius as well methinks. It might sound smug, but it's really why we bought the car, to save the planet. I want people to know that they can do it as well.