My back of the envelope calculations give me around 500 gallons of fuel just to gain that kind of elevation (not taking into account anything but difference in potential energy). I assume most flights are less than 500 miles (I fucking hate that I had to do all of this in imperial) based on what I remember seeing from that Twitter account that used to post 's private jet. In order for that fuel efficiency to be true cruising at that altitude would require basically no fuel or even negative fuel.