> Fuel efficiency dives as you approach the speed of sound, airliners were already going as close as you’d want to in the 70’s. They were, in fact, going to cl

> Fuel efficiency dives as you approach the speed of sound, airliners were already going as close as you’d want to in the 70’s. They were, in fact, going to close to it. Which is why they now cruise at lower speeds. > Since then planes have slowed down slightly and overall fuel efficiency has gone up considerably. Absolutely.