I said most modern cars e.g. built in the last ten years will give you optimal fuel economy in the 70-75 MPH range. Everyone gears it that way and most modern engines can pull it off. Except for grossly under-powered vehicles, it really does not matter what you are driving. I was just giving an example I am familiar with because I've had more than one car in that power range with detailed real-time fuel efficiency data.
Seems to me most "cars" on the roads are pickups, minivans, and SUV's... I would like to see a fuel efficiency test redone with newer cars.
You'll have to prove that. Wind resistence increases geometrically, so I'm not buying it. It takes considerably more energy to push a car at 70mph than it does at 50mph. I'm not buying it.