A vehicle at 85mph will use about 40% more fuel than one at 70mph. That means it costs more, it uses up a finite fossil fuel faster, and causes more pollution and CO2.
In both the car's I've owned, this is not the case. Other factors, especially gearing (controlling where in the power band the engine is running) have a huge effect. My current car (Pontiac G6) gets about the same mileage at 80 MPH as it does at 70 MPH. My previous car (Pontiac Grand Am) got better mileage at 85 than at 70. Yes, you're burning more fuel per unit of time, but depending on horsepower curve, your fuel per unit of distance may actually be the same or lower since you're covering more ground in that time.
Because of aerodynamics, the wind resistance curve will at some point always win out and drive that efficiency curve downward, but you can't universally assume the peak of that curve is below 85.
Both cars I had were mechanically fully capable of 90 MPH+ cruising speeds, as long as the driver were able. The reason that high speeds are dangerous in the US is because (as stated elsewhere in this thread) drivers here are not very good at it. I've long thought a country of this size would benefit from a high-speed toll road with special licensing and vehicle requirements. It would work fine as long as lane discipline and safe following distance were enforced, like on the Autobahn.
Things like staying in the left lane without passing, driving too slowly, making unsafe lane changes, inattentive driving, etc. are already illegal and enforceable in most if not all jurisdictions, and I wish cops started paying equal attention to these drivers that create problems and not just speeders.