...as you don't take my word...
from 330/340 FCOM
CI is the ratio flight time cost (CT) to fuel cost (CF).
CI = CT/CF KG/MIN or 100 LB/H
The cost index is used to compute the best economic speed and Mach to be flown
considering the ratio between the cost of the flight time and the cost of the fuel.
CI = 0 corresponds to minimum fuel consumption (max range)
CI = 999 corresponds to minimum time
It is recommended to modify the CI in flight:
. In case a fuel problem is encountered, CI = 0 may be selected; the ECON SPD profile
is then computed to ensure minimum fuel consumption.