Just to inform those who do not fly a 747 "Classic". long range cruise for heavy weights starts at close to M.860 and high twenties. So M.855 and FL290 combination is actually very efficient. If you take a careful look at lft CDU you may observe that a step to a higher level is less than 10 minutes away and they are only slightly below optimum.
Efficiency has never been a sole fuel consumption figure, rather a combination of various factors such as schedule, importance of load, ATC etc. Cost Index and way it works gives a pretty good idea of what is reuired to keep a flight efficient.
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