The MPG in my car is pretty accurate. Can be disappointingly accurate at times. Calculates distance travelled to remaining fuel volume, and you can change the settings to go into a little graph with the consumption levels over a specified time, or choose certain timeframes so you could for example check how you did overall on the first hour of the journey compared to the second hour.
I don't know of any aftermarket measurers.
I was just writing a big long paragraph about how one could do it, but now I've decided to delete that and put it into my "business ideas" book :P Thanks for this thread, I'm excited now!