Hi,   COSTS INDEX  

The airlines instruct the crew about the costs index. It may be the same for every flight or could vary from flight to flight.

Two major components.   Fuel costs and operational time costs.

The two extreme are  

0   will result in maximum range and minimum trip fuel.
    but will ignore the costs of time.

Conversely a Max entry, the FMC  uses the minimum time at Max speed and will ignore the costs of fuel

Airliners fall in between these two.

I fly at econ   60, it gives me a speed of 301 and a M .796
This number is realistic for a large Boeing-twin engines.

On long haul flights, I see airplanes come home 2 hrs late each flight
at M.82 -save a lot of fuel at the end of the year.  ;D

Raymond   Boeing or nothing
KFLL   South Florida  8-)