Any head wind, even a slight breeze, will have a relatively large impact on your mileage.
I drive 62 mph - 65 mph, as already noted mileage starts to drop noticably at speeds higher than 65 mph. If I keep my foot out of it I routinely average 12 mpg, have seen a bit higher when I go east into the flat lands but that's pretty rare as there is usually a wind (and the wind always seems to be a head wind no matter which way I go).
Best mileage I've ever had on a single trip was in the prairie, no wind, averaged 13 mpg.
Worst mileage I've ever had on a single trip was in the prairie fighting a big head wind between Lethbridge and Calgary. I averaged 7.5 mpg on that one.
Overall I typically average right around 11.5 mpg.