Recall a couple of weeks ago we had the report here of how Italy was raising its highway speed limits to over 90 to improve safety?
Re fuel ecomomy, it is interesting that in my old Datsun 260Z, the best gas mileage I ever got (over 30mpg for 2 consecutive tanks) was during a high speed tour of western Colorado and Eastern Utah -- extended periods well over 100 mph. At speeds closer to the then 55mph limit, I usually got around 25mpg.
I had a similar experience a couple of years ago in my Audi A4 1.8. I was returning from SoCal to Park City on I-15. Just past Cedar City, some guy blew by me in his Lexus, so I figured I'd let him run interferrance with the Utah Highway Patrol. Let him get 1/4 - 1/2 mile ahead and then matched his speed, while keeping my ears open for my radar detector buzzer. For the next approximately 100 miles, our speed was rarely under 100, and several times I hit the electronic speed limiter (130, per Road & Track, about 135 by the speedo). That tank of gas, I actually got better mileage than on the previous one, where the cruise control had been set at 80.
I am not suggesting that faster speeds are necessarily safer or more economical, but the generalization that lower ones are safer and save gas are not always valid either.
Your point about rural western interstates is an important one. In many cases, the only limitation to what is a safe speed is the vehicle itself. Christ, I recall in the early 60's, the car magazine testers routinely would test their exotics (Ferraris etc) in the Nevada desert at 180 mph, perfectly legally, and probably very safe. As long as the vehicle is designed for stability at those speeds and tires are made to handle it, there is very little risk.
Even 75 mph limits on these roads are ridiculously low.