There are two optimal values for speed depending on the objective. There is a max endurance speed (for holding, etc.) and a max range speed, both are a function

There are two optimal values for speed depending on the objective. There is a max endurance speed (for holding, etc.) and a max range speed, both are a function of altitude. So slowing down under those speeds will decrease efficiency, but typically the airline's cost index (~additional hourly cost to operate, like crew cost) will typically push the speeds higher to minimize the overall cost. http://www.eaa1000.av.org/technicl/perfspds/perfspds.htm http://www.eaa1000.av.org/technicl/perfspds/perfspds.htm is about the best thing I can google right now as a reference.