Thanks for sharing, WT. Because of the number of variables which come into play to result in average values, though, it becomes difficult to translate such to individual situations. If we as a community could obtain even one set of performance curves for each engine type as outlined in the 'Horsepower and Speed' thread [
http://x3.xbimmers.com/forums/showthread.php?t=1004702 ], we can at least have a rough reference to which we can refer. I can produce summary performance graphs for each engine and then graphs per single variables (such as 'mpg vs engine' type) if such data is shared. Without a number of entries the data will understandably be biased, but it would be a start.
This would then allow a broader range of information use across the board it would seem. There may come a time when one is in a rural setting away from a fuel source with our tanks lowering when it might be helpful to hit each engine's 'sweet spot' for maximal mpg. I suspect it would be in the 45-50mph range, but other than what I posted for our X3 35i 2012 and 2015 vehicles, I haven't seen anything else online to support such for the other engine configurations.