Many vehicles have a computerized system that displays the ratio between fuel consumption and distance traveled.
This is somewhat useful - as far as it goes - but doesn't take into account a number of factors that influence the resultant value; the terrain over which the vehicle is driven, the driving
style, and the amount of other traffic - more applicable to an urban setting.
What the user really wants to know is how much "value" they are getting from their fuel - is the engine operating efficiently ?
Since modern vehicles are extensively instrumented, it seems reasonable to use the data to provide a better metric. Distance traveled becomes irrelevant; what matters is how much energy (in MJ) was extracted from a given amount of fuel.
The torque delivered to the wheels can be measured, as can engine revs, and vehicle speed. These data, suitably integrated, reveal how much energy has been used by the vehicle.
The amount of fuel consumed to deliver that energy can also be measured (within normal error bounds).
Thus, the system knows what the fuel usage was to deliver a known amount of energy as usable propulsion. Even an engine simply left idling will give a figure for "fuel consumed to maintain nil usable energy output".