Okay, here are my findings so far. Do these sound reasonable?

Assume 800 miles in one day.
Assume $3.00 per gallon of gasoline.
Assume 1.5 hours per trip for food, fuel, and potty breaks.

800 miles @ 75 miles per hour = 10.6h + 1.5h = 12.1 hours total trip time
800 miles @ 70 miles per hour = 11.4h + 1.5h = 12.9 hours total trip time
800 miles @ 65 miles per hour = 12.3h + 1.5h = 13.8 hours total trip time

Slowest vs. fastest delta t = 1.7 hours ( cost vs. savings )

800 miles @ 75 miles per hour and 27 MPG = 29.6 gallons FEH
800 miles @ 75 miles per hour and 36 MPG = 22.2 gallons PriusII
Delta fuel = 7.4 gallons Delta cost = $22.20

800 miles @ 70 miles per hour and 31 MPG = 25.8 gallons FEH
800 miles @ 70 miles per hour and 45 MPG = 17.8 gallons PriusII
Delta fuel = 8.0 gallons Delta cost = $24.00

800 miles @ 65 miles per hour and 34 MPG = 23.5 gallons FEH
800 miles @ 65 miles per hour and 50 MPG = 16.0 gallons PriusII
Delta fuel = 7.5 gallons Delta cost = $22.50

Assume 4 trips per month:
Total Cost Per Month @ 75 miles per hour = $355.20 FEH
Total Cost Per Month @ 75 miles per hour = $266.40 PriusII
Delta Cost Per Month @ 75 miles per hour = $ 88.80
Delta Cost Per Year @ 75 miles per hour = $1065.60

The delta cost for owning a FEH vs. a PriusII for ALL HIGHWAY USE
based on a very vigorous 3200 miles per month, or 38,000 miles per year is
$1065.
Most people will drive closer to 19,000 miles per year.
Based on a more reasonable 19,000 mile year, the FEH costs about $530 more fuel to use than a PriusII.

What I also walk away with from this data, is the delta cost of ownership was consistant with changes in speed. Thus, I conclude from this, neither the FEH nor the PriusII is better or worse suited for highway use. The delta cost stays the same between 65 miles per hour and 75 MPH, only the total cost changes.

The Prius' shape does not look that much better, and the FEH shape does not look that much worse from an aerodynamics fuel standpoint, which suprised me. If you see fault with my data, let me know.

Comments? Thoughts?
-John