In their readership survey of Durango owners about a dozen or more years ago,
Popular Mechanics magazine reported that the owners wrote that the 360 apparently got
0.75 MPG less
than the 318

It is so much easier to get MPG increases
with low rolling resistance tires,
And simple aerodynamic improvements
That those two areas should not be overlooked.

Improving innate engine efficiency is hard...
But getting to a better area of the "island" of best efficiency on the BSFC graph
Is mostly about using a numerically lower differential ratio,
And perhaps selecting a camshaft to match the rpm range of whatever diff ratio you go with.