[serialk11r]

Drag drops proportionally to air density, throttle losses on most engines are something like 0.8 * vacuum in BMEP, so if you have like 0.5 bar vacuum at cruise and go to 5000 feet where the vacuum drops to about 0.3 bar, you're saving 0.2 bar of specific torque.

If that used to be say 4 bar specific torque, 1 bar worth of friction and throttle loss (say half and half), and 1/4 of the energy went to rolling resistance now you only need 3.4 bar to maintain the same speed, friction is still about 0.5, but throttle now consumes only ~0.4ish, so you're looking at 10%ish less fuel.

For a shorter geared cruising gear, you might see a bigger % of throttle loss, but also a bigger % of mechanical friction loss to pair with it, so it could come out around the same.