[namirez]

You are right but the reality is even worse. Fuel consumption is proportional to power (velocity * drag) rather than drag itself. When you go twice as fast, the travel time is halved, so the total fuel consumption quadruples.

And as you said, supersonic drag coefficients are typically larger than subsonic ones, which makes things even worse.

[jpeanuts]

Thanks for correcting my embarrassing error! It's too late to edit my comment, but indeed x4 is the lower bound.