Been there, done that.
Actually, I've put the cruise control on 60 mph. Let me tell you what difference it has made.
Zip. Zilch. None. Nada.
My combined city/highway mpg is 18 mpg. No matter if I drive 60 or 70-75 mph, it's all the same difference.
Why? because today's fuel efficient vehicles are engineered to maximize better mpg's at average highway speeds.
I hear you...however...
not all cars out there are going to be so well engineered...my 2000 chevy blazer has been giving me nearly 60-80 miles more per tank by going 55-60 mph for the most part when I can...so instead of filling up every 7 days it’s moved to aboutevery 9 or 10 days so I am saving money, albeit not a ton, but money saved is money saved...if it wasn’t we’d all pay top dollar for everything and not care.
That being said, I in no way would support 55mph national speed limit...local and state decisions like that should be local and state...65mph on interstates needs to stay as well...
B.S. It takes more energy to go a faster speed on average. Try peddling your bike at 20 and then 30 mph to see what I mean. However, mainly because of the non-linearities involved in the transmission of the vehicle, as well as programmed changes to timing and injection, and dynamic changes to load, it is likely the efficiency curve is also non-linear with some spikes and transitions. Therefore you could probably find two adjacent speeds, say 56 and 57 mph, where the higher speed actually has lower fuel use. But between 60 and 75, no way. An inline fuel flow meter would make that very clear.