Reducing the fuel/air mixture doesn't increase efficiency, yes you need less fuel per unit of air, but you need more units of air too. The problem is the air g

Reducing the fuel/air mixture doesn't increase efficiency, yes you need less fuel per unit of air, but you need more units of air too. The problem is the air gets thinner as you go higher, meaning you need to fly at higher speeds, which means you need to fight more drag. Really it's not flying at low altitude to save fuel, it's flying slow to save fuel. Jet engines become more efficient at higher speeds which compensates for the increased drag, and thus jet aircraft fuel efficiency goes up at higher altitudes. Piston engines are unaffected by airspeed, so at higher speeds they are doing more work at the same engine efficiency. Thus the aircraft fuel efficiency goes down.