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.
Actually most cars and LD trucks get the maximum FE at 35 to 45 MPH. It's just that no one drives that speed for hour after hour on the freeway to get the data. Most driving is done at 35-45 and includes lots of stop and go along with short trips, neither of which is conducive to good FE. Thats why hybrid vehicles get better city economy than highway economy - they are designed for the stop and go traffic of city driving.