I drive my GSW to work and back for about 120mi, 115 of which on the highway. I drive 60 to 65 on the 55mph limit stretch, and 70 to 73 (rarely over) on the 70mph stretch (2/3 of my whole commute).
I DO get 50mpg+ (53 is not a surprise) consistently in fair weather (above 55F, no headwind, rain etc). Any meteorological event can hit my mpg significantly, and it happens not so infrequently. For example, water on the road (even shortly after a downpour, not necessarily during it) can make me drop below 47mpg. Wind, even worse. Frigid weather in winter also hits my mpg significantly: in otherwise fair conditions except cold, I get maybe around 47mpg if it's in the 30's, 45mpg or worse if it's in the low 20's or single digits.
I hand calculate my FE. I also used OBDII to recalibrate the displayed FE by -7%, so now it's almost spot on (maybe +- 1mpg discrepancy).
Also, my route is not even close to flat terrain (southwestern PA, northern WV). I'm positive I'd do better on the flats.