[Big Dave]

MazdaMatt noted:
“…Lower TP at the same RPM puts your engine in a less efficient point on the BSFC chart?”

Big Dave comments:
Absolutely true for an Otto-cycle engine. Maybe not true for a diesel. Diesels do not have throttles and hence have flat torque curves and very large BSFC ”islands.”

Even with a gas engine, sometimes the loss of BSFC is less than the overall fuel burn rate so you still get an improvement in MPG, just not as much as the proportional reduction in drag would indicate.

This increase in BSFC is the nugget of truth in the old urban legend of driving at higher RPMs gives better MPG.

If you can achieve a reduction in CdA, then the correct follow-up is to re-gear to keep yourself in an efficient range at the desired road speed.

As I keep saying, gearing is (like aerodynamic improvements) a powerful tool for the real seeker of MPG.

Larger diameter wheels/tires are counter-productive and by no means a substitute for changing gearing. Larger diameter wheels add heights and frontal area and worse yet, add rotational inertia. If bigger wheels improved MPG, then all the "Bigfoot-Wannabe" lifted trucks should get terrific MPG.