You are doing a lot of assuming. I did not spend much time driving at 55. I did spend a lot of time sitting in construction zones, fuming at the delays, with engine and a/c running but either stopped or barely moving. When not in a construction zone the cruise was set at either 75-77 or 82-83 mph, depending on the speed limit. Matter of fact, after the trip (3000+ miles) was over and I checked my GPS unit it told me at some point I had hit 93 mph.
But that's not the point I was trying to make. I was driving a truck that essentially has a 2000 lb payload, yet it was getting fantastic fuel mileage compared to any pickups I owned in the past.