Guy's
There seems to be a modicum of confusion here w.r.t. efficient operation of modern jet transport a/c. I think the question you have to ask is what are the operators/manufacturer's specifically trying to achieve - even someone as academically challenged as myself can work out "lowest overall cost"!
Operating a jet transport a/c from A to B for minimum fuel burn involves climbing at best lift/drag ratio speeds (Vy) into the rarefied atmosphere, where both airframe efficiency (IAS/TAS) and engine efficiency (SFC) is greatest. One would then remain there until the point at which a descent, again conducted at best lift/drag ratio speed (Vy), is required to arrive at destination. This is (or very close too) the profile that an FMS would furnish with Cost Index 0 (very high fuel cost).
However, is this what operators are trying to achieve - no. Other costs, such as crew, maintenance and a/c utilisation have a large part to play. Remember that fuel is relatively cheap! Thus, over all ECON speeds are a compromise between fuel burn and airborne time, and are defined by the Cost Index that we enter into the FMS. The result will be climb/descent speeds in excess of Vy. Try popping a CI of 999 in the FMS next time you fly, and look at the predicted speeds for climb/cruise/descent! Makes for a very attractive flight time, at the expense of a lot of fuel.
Of course, much of this is rather academic, as we have to vary our speeds to suite various restrictions/ATC. Reverting to the cost index 0 techniques is, however, a very handy trick to have up ones sleeve should you find yourself becoming rather tense about your fuel situation
Regards,
Cuban_8