[Johnny Mullet]

With mostly highway miles, aerodynamics and tire pressures will net the most gains.

If it were my car, I would...................

Block off some of the grill and monitor engine temps and adjust accordingly.
Raise the tire pressures to max posted on sidewall of tire.
Construct an underbelly pan from some kind of material (free stuff works best)
Remove the passenger mirror (if legal) and relocate radio antenna.
Smooth wheel covers.

Good luck!