The following is my recollection of listening to an SAE engine designer talk about HP/MPG/Mapping.
Every engine has a map of the engine abilities, efficiencies, and how it uses fuel. Inside the map is a bubble of area that you can optimize. The factors effecting it is the type of fuel, the delivery mechanism, the structure of the engine, the transmission type, the gearing of the engine, etc... If you have an engine map you can move the factors (vectors) around within the map to optimize.
So, I'm trying to get access to the engine map for a 5.3, and 8.1 just for grins and giggles. I'm imagining the map will determine what the max (theoretical) MPG of the engines can be, and what vectors have to be effected to get there.