In simple terms MRC is the speed one would fly when desiring "max range" in relation to fuel consumption. In theory LRC provides a slightly less fuel efficient cruise but provides better speed stability in average conditions.
Most Flight Management Computers have the ability to select LRC as a cruise mode. Note that LRC will not change when the cost index is changed. To determine MRC on these aircraft simply input a cost index of zero into the FMC. Econ Cruise is then MRC. Note that LRC does not change. So far I have not used MRC operationally but it's nice to know how to find it and, or use it.
Cheers.
[ 17 December 2001: Message edited by: Clipper811 ]