[twindman]

From my classroom Physics days, ahem, wind resistance is equal to the square of velocity at some speed. Go faster, it is the cube and real fast it is 5th power. I think those higher numbers would involve speeds over 100 mph or higher.
On a more practical note, I recently pulled across southern arizona and new mexico. I got around 8 mpg, if I recall until about noon. That is where we changed from a SE direction to straight East and also noted that the wind died down a bit. So the next 200 miles I got around 10.5 or so. So for a flat highway without stopping, I vote for wind resistance (directly related to speed plus actual wind). Of course in town withOUT towing, I only get about 15 doing stop and go traffic - even if I use the old 'eggshell' under the foot method. Every stop and start lowers the mpg by 1-3 points.