Go back to basics.

If 3 otherwise identical PowerWagons are driving on a highway at 40 mph,
all of the same weights,
tires and gearing,
but one has a 273,
one a 318
and one a 340 V8
why does the 273
get better MPG at this steady speed situation?

The answer (mostly) has to do with MAP
(manifold absolute pressure)

Is a picture worth a thousand words?