[albrewer]

You're ignoring the efficiency of the jet engine itself. You have to basically pick an altitude, temperature, and speed at which the engine is most efficient. Modern airliners are most efficient at a certain speed and altitude because they're designed to stay under the supersonic regime.

It's been a long time since I've touched compressible fluid mechanics or turbomachinery design, but if I recall correctly the theoretical "sweet spot" for overall aircraft efficiency is something like Mach 1.3. I'd have to dig though my textbooks to remind myself why, but the number stuck with me.

[Gravityloss]

Definitely! I remember seeing an old picture of a physical three dimensional plot about theoretical airliner efficiency. X axis was speed, Y axis was something else and Z axis was efficiency. I think it was made of wooden shapes. There were two local maxima there, one subsonic and one supersonic. Probably it was related to the US SST project.

Can't find the picture anymore...