Just to clarify, MRC is a defined speed that provides maximum range in relation to a given amount of fuel. So "ECON" is only equal to MRC when the cost index entered is zero. When zero is entered one is telling the FMC that the only "cost" consideration for cruise is fuel. Hence "max range cruise." If something other than zero is entered (into the cost index) one is telling the FMC to consider "other" costs such as crew, mx, on time dependability, etc. Again, the reason there is a LRC mode available on the FMC vs. a MRC is that the engineers think it more advisable to fly LRC for better speed stability when fuel conservation is the priority.