Fellows,
This guy in India sells a system consisting of two diesel fuel flow meters and a LCD digital display that calculates the difference of the two, for net fuel consumption (liters/hour).
The only thing missing is to take speed in km/hour and divide by consumption, and we will have fuel efficiency in km/liter (or miles per gallon).
The result may be shown in the same digital display.
Look here:
Fuel Consumption Meter - Diesel Generator Fuel Consumption Meter, Automobile Fuel Consumption Meter, Vehicle Fuel Consumption Meter and Fuel Consumption Calculator Manufacturer & Exporter from Mehsana, India
I have asked for a quote but no response so far.
Oldbeaver