There aren't any stock gas or diesel vehicles that can do 40 mpg at 75 mph.
My BMW does 40mpg @ 90mph and about 50mpg @ 65mph. I believe it's because of the efficient diesel engine and the low aerodynamic drag coefficient. The undercarriage of the car is almost completely flat, except for the wheel arches and the rear end where the rear muffler comes out. The grille is blocked by something similar to window blinds that automatically opens if the engine needs to be cooled. I think the Cv overall is very low for the body for a normal looking car.
This is with an automatic transmission. There is an EDE (Efficient Dynamics Edition) with only a manual high geared transmission that does even better.
I had a 1991 Chevy Cavalier 2.2L 4 cylinder and 5 speed manual that would get 35 mpg at 60-65 mph, but would get 42 mpg at 80 mph. I used to know several others that got similar results as I had at similar speeds.
Come to find out, the 2.2L motor and 5 speed combo sits right at peak torque when cruising at 80 mph (2700 rpm or so).
The 3.1L V6 doesn't get as good of fuel economy at those speeds, but is still in the mid 30s which is still pretty good.
I just added this to my site as one of the "Testimonies" to keep in mind. I am new to this but am looking for proof that our cars are woefully inefficient and that inefficiency has been shelved by car producers and oil companies...Once I know this for a fact...maybe I can inspire others to act with the knowledge. I will be trying things on my personal vehicle but what we really need is a national uprising
Car manufacturers only make cars people are willing to buy. As long as the cost of petrol is a fraction of the total running costs of a new car nobody is going to care unless money is not an issue or the environment (ecology=long term economy) is more important on a personal level.