[Sector-9]

I think there are just too many variables to be able to calculate fuel consumption solely from engine capacity and speed. Just as one example, the electrical loads are never constant - even the battery's state of charge and temperature will affect the loading from the alternator. Invariably they measure fuel usage either through a flow meter (as in the old carburreted Sierras) or count fuel injection quantity per stroke (as rail pressure and injector flow rates are relatively fixed known values), and if you think about it, even the fuelling map for the ECM to decide how much to inject is only based on a limited number of sensors and relies on driver adjustment (the accelerator pedal) or cruise control to compensate for the unmeasured loads.

This is one of the things that make more modern engine management systems use less fuel and burn cleaner: they have a greater number of sensors measuring things like electrical loads, and actuators that can respond accordingly such as reducing alternator output when possible. Whether the benefits are worth the complexity and likelihood of faults occuring is a different matter...