Originally Posted by freshairborne

The short answer is, finding the right combination of speed, altitude, route, tailwind/headwind that gives the lowest cost block-to-block flight.
Fuel burn is only one factor, though. On a zero-wind, standard atmosphere (never exists, but you gotta start somewhere) day, an airplane has it's favorite minimum fuel flow altitude for any given weight and cruise speed. Generally, a lighter airplane is more fuel efficient at a higher altitude than a heavier one. If there's more tailwind or less headwind up higher or down lower, it might be worth the extra fuel flow to go get it, or get out of it.

Outside air temp plays, too, and the height of the tropopause is a factor, too because there is generally worse rides when within 4,000 feet below, and 2,000 feet above it.

Airframe time costs $$$, so we consider flying faster than optimum miles-per-gallon speed to offset that cost.

Now comes the tricky part: how much gas do we save without piʂʂiɲɠ off your customers? My feeling is that you become my customers once you're in the plane, by the way. do we step on it to make a few connections at the expense of 1,000 pounds of gas? Do we find a smooth altitude even though it's got more headwind, or deviate an extra 50 miles around some bumps, so you don't grumble about the crappy ride and the seat belt sign being on a lot? Will you fly another airline if you think we should have done these things? How many barf bags got used because we didn't do this?

Truly, a little extra fuel, used properly, buys a lot of customer satisfaction.

FAB