My mistake. MRC is .826 LRC is higher.
We use cost indexes as low as 30. Highest I recall was about CI300. Difference in cruise mach is about .83 to .843.
@Charterjake - I engage ALT HOLD and speed intervention and enter other mach speeds into the FMC. It shows the time and fuel value of the changes. I'll play with the numbers again on my next trip.
I've yet to see a flight plan planned at .85. It's either .83 or .84 depending upon flight plan vs. schedule. Makes me think the company's decided the fuel cost of .85 isn't worth it.