My speed depends upon the road conditions and traffic. In California and Nevada the truckers are usually driving at 65-75 MPH and the people pulling 5th-wheel trailers are doing the same. The folks in cars and pickups are driving at 70-85 MPH. It would be stupid to drive at 55 MPH or even at 60 MPH in those conditions and it would also be irresponsible and cause others to do a lot more lane changing and this is dangerous.
I also do a simple calculation of my speed, the distance of the day's driving, and the time I might save by driving 10 MPH faster. Going 200 miles at 75 MPH versus 65 MPH is going to result in arriving at the destination only 24 minutes sooner. What is common and crazy are all the motorists making trips of 30 miles or less where their speeding gets them to their destination at most 5 minutes sooner - how dumb is that?
With the camper fully loaded and no head or cross wind my diesel powered truck gets the best fuel economy at between 62 and 65 MPH. The fuel economy is noticeably poorer at 55 MPH and it is no doubt that the engine is outside of its peak power area when the transmission is in 6th gear. Fuel economy is even worse at 45 MPH with the truck with or without the camper for the same reason.
With a 15 MPH headwind the fuel economy drops by 15% same as if I was driving at 80 MPH down the highway. That would also translate into paying 15% more for fuel. Not sure how other people do the math but on a 1100 mile trip last month through the sierras with the camper the truck got 11 MPG and fuel cost a little under $400. Had I driven faster and lost 15% in fuel economy that would have added an extra $60 to the cost of the trip and in the overall scheme of things that is trivial.
Bottom line I drive at the speed that is appropriate for the conditions to maximize my safety and that of my passengers and to avoid conflicts with others on the highway. This last concept seems to escape most American motorists where there is a sense they have special rights and priviledges and would sooner die than let someone safely merge into traffic if that means they are going to be in front of them. And it is peculiar to drivers in this country.