[landsailor]

The wind speed here can be steady 10-20 mph with all day intermittent gusts to 40 to 45 mph. Check out the National Weather Service, Grove, OK.,or 74354. Spring and Summer usually out of due South-Southwest. Winter mostly Northwest to due North.
I drive a due South and due North trip nearly every day round-trip. At 55 mph I get my poorest mpg either way in even calm air(rare).
The FIRST factor is the BSFC. I have a 1999 Mazda B-2500 2.5L 5 speed (Ranger clone). BSFC range is 2000-2400 rpm. 55 mpg drops me out of this range. MPG via Scangauge plummets to 22-24 mpg. 10-15 mph headwind at 55 mph is 18-22.
The SECOND factor is temperature. Both outside temp and engine coolant temp. If I get into the 2200-2400 range with no wind, then 34-38 mpg(outdoor summer temp's 80-100 degrees, water temp's 198). Same conditions with winter outdoor temps( 0-50 degrees) then 22-26 mpgs with a fairly air-tight full grill block. During winter outdoor temps, the thermostat doesn't even open at it's 191 degree threshold(coolant hovers between 174-184).
The best mpg's are summer outside temps and 194-200 degree coolant temp with tailwind. 10-20 mph tailwinds can result in 38-44 mpg's.
Hailstorms and severe cloud to ground lightning result in 0 mph and 0 mpg under the nearest roof or overhang and wishing I had a lower deductible.

The above is influenced by some extensive front to rear bumper aero-modding and a cab level bed topper. Will post some pic's when I have enough post's.