The bunching of traffic caused by 55 mph speed limits was endangering people's lives. Moreover it's far from obvious that anything other than the stodgiest vehicles were getting max gas mileage at 55. Most are going to get optimum mileage at the point at which valve timing becomes optimal in top gear and that in most modern cars is going to fall around 65 - 75.
Again, very bad science. Where do you people get this stuff?
Again: Drag is an exponential force. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Since power is the rate of doing work, exerting four times the force at twice the speed requires eight times the power. So, until they design an engine that uses less fuel while generating 40hp than it uses while generating 100hp (a physical impossibility) your statement is going to be absolutely false.