The speed we use for cruise for 737400 type is the ECON speed selected from the FMC.There are factors involved for the calculation of the econ speed which varies from operators.(cost index,weight etc).
When I select the ECON on cruise -400 gives M.74.I can also change it by either entering different value on the FMC or changing the cost index.You can also control the speed from the MCP
When I fly the 737-800 the ECON gives M.78 but often we fly M.80.(Thats because my company is a small one and every airplane has other flights after our arrival.If we are in a delay for some reason we try to make it as quick as possible back to base.The difference of extra fuel burn will compansate the loss due to delay)By the way I have never seen a 737-800 flying with M.81 before, anytime I write .81 to the FMC it gives back the message "UNABLE MAX M.80.5"