(Denny Crane: "Sometimes you can only look for answers from God and failing that... and Fox News".)
I drove a 5.7l V-8 in Montana when the speed limit there was "safe and reasonable" (due to a court case they had to implement a specific limit a few years ago). I probably averaged 100 mph outside the cities. My mileage? 25 mpg, exactly what my car was listed at. And my car, it was like a big dog that finally gets a chance to get out in the open fields and run, the engine was purring.
As others have noted, the notion that cars get maximum fuel efficiency at or around 55 mph is simply asinine. The cars I had were most fuel efficient at 70-80 mph.
Only a Manhattan (NY, not Kansas) resident could come up with something so utterly ignorant.