Haughtny,
It defends on how you DEFINE economical cruise.
economical cruise on a 727 is around 78-80, if fuel is your consideration. FEDEX (and Ryan international when we flew the mail) fly the 727 considerably faster (.84 - .90 depending on how much time needed to be made up) than that and very close to the barberpoll when time is of the essance.
Its a whole different animal than pax flying. Speed is life. If a package isn't there on time its FREE. If a passenger is late, who cares, they already paid.
Cheers
WIno