Say what you will. At 55 mph, I can go about 14.3 miles on a gallon of gas. At 65-70 mph, I can go just over 15-15.2 miles on a gallon of gas. I’ve measured this based on the same course, same time of day, etc.
Yeah, I'm sure our engines are designed for optimum performace at out driving speeds.
On the other hand, going from 55 to 65 is not an appreciable difference in speed. Ignoring eveything else, drag at 55 would be proportional to 55 ** 3 (166,375 somethings), while at 65 drag would be proportional to 65 ** 3 (274,625 somethings). That means that energy loss at 65 mph due to drag is 1.65 times as much as at 55 mph. Double the speed, and air drag would figure very prominately. (1,331,000 somethings, or 8 times as much.)