For a start...
. OPT ALT is based on gross wt, speed entry, and route distance.
. ECON is based on lowest operating cost per nm.
. LRC and SEL SPD, fuel used per nm is minimized.
. For short range flights, this altitude allows for cruise time requirements.
Ref: PERF FACTORS
Pegasus RECMD...
. Recommended altitude is most
economical altitude based on two
engines, gross weight, target speed.
forecast cruise wind and temperature.
If no forecast winds are entered the
FMC uses sensed winds
. May be above or below cruise altitude