Flumoxxed or Bamboozled ? : On board MPG calc.

Not sure about cars, but my Workhorse chassis measures the fuel being used, and calculates an instantaneous MPG value or an average value - the average being a rolling average over the last 50 miles driven. Instantaneous value is pretty much useless except for training your right foot to be light. I leave mine set on average and it shows a reasonably accurate reading that changes with the terrain and driving style, but gives a pretty accurate reading so you can plan fuel stops, etc.