decurion:
Note that a CI=0 equals MRC.
That's only half the truth. MRC is a fixed speed for a given weight, ECON speed is not.
With a strong headwind ECON (with cost index 0) will fly faster than actual MRC and LRC for that matter. Since we have no representation of actual MRC, the practical application would be LRC.
The reason being, that ECON will take actual wind into consideration whereas LRC doesn't. From practical experience even a Cost Index of more than 10 sometimes give a lower speed than LRC if you have a strong tailwind (FMC deems it more economical to hang around in the tailwind for a little longer and giving us a free ride).
If fuel is critical, I would suggest to compare and consider the options of LRC and ECON CI=0, if headwinds conditions, before deciding on the better option. Being stuck in a strong headwind at LRC may give you less fuel at destination than letting the FMC do it's magic.
Strangely you wont find that information in the FCOM, but in the FCTM under "Cruise Speed Determination":
ECON cruise is a variable speed schedule that is a function of gross weight, cruise altitude, cost index, and headwind component...
..Headwinds increase the ECON CRZ speed. Tailwinds decrease ECON CRZ speed, but not below the zero wind maximum range cruise airspeed.
In the same section you also find the that for NG, wind correction is
not applied for LRC:
LRC is a variable speed schedule providing fuel mileage 1% less than the maximum available. The FMC does not apply wind corrections to LRC.
(Strangely it
does apply wind corrections to LRC in the classic, thereby not being a "true" LRC according to the definitions).