Canuck, please excuse my typo, I meant to say Vmd was effectively max endurance which is ostensibly max range (I was waiting for someone to spot the deliberate mistake
)
In the FCTM for the Boeing I fly it says "entry of zero for cost index results in maximum range cruise".
Given that the total drag curve in this region is effectively flat, the speed differential in terms of IAS at representative cruise alts and weights between holding speed and CI 0 is 1-2 knots on the jet I fly, which brings us back to the problem of speed stability. Yes in theory CI zero gives max range, but in practice the fuel predictions slowly reduce to the point where they would have been if you flew .05 Mach faster.
The only reasonable explanation I can come up with is that it's thrust adjustment to maintain an unstable speed that causes the increased fuel burn.
In the military, the wingman flying formation always burns more gas than the boss
