I belive he's trying to compare the operating ceilings (not sure if that's the right term) and max crusing machs for different aircraft. I'm sure other people have much more detailed data, but I remember something in Airways magazine a while ago where they had to correct themselves that the 747 did not have the highest crusing mach, rather the 727 did. That is, of course not including the concorde! I believe it was mach .83 that the 727 could cruise at as opposed to the 747(-400)'s mach .82, but my memory is very sketchy. I'm sure someone can add better details.