Originally Posted by barit1
But with less total thrust available, a lower cruise altitude is forced on the machine, which ruins any advantage. The higher air density means more thrust available, but more fuel burned too.
Which is probably because the jet engines are less efficient at lower TAS and therefore generally less efficient at lower altitude. If a plane descends below the optimum cruise altitude for a jet then I suspect that whatever the plane does - running all engines at less than optimum thrust, or shutting some down and running the others at the optimum thrust - the plane would be less efficient in burning more fuel per distance, and therefore lose range for a given fuel reserve.