One more clarifying question about this:
Does it matter whether this is done prior to T/D or after it starts a descent. One person I talked to believes (and maybe correctly) that the first speed constraint is only created if it is done during the descent and that it will still go up to VMO-11 if you insert a descent speed while prior to T/D.
In other words, if during cruise I replaced the ECON DES speed in 2L of the descent page with something like M.840/310, would it have any different effect if I did this during cruise as compared to the effect of doing this during the descent (while the descent page is active)?
Would it still be limited to FMC command speed (310) + 15 knots in either case?
Thanks!