Just looking at your plots, I suspect the dip you see around 40 mph is the result of timing retard. Most likely it is mapped to provide maximum resistance to detonation at that rpm against that kind of loading, whereas just the drop to 35 mph reduces the drag loading substantially, allowing less timing retard to essentially just put along.
In other words, you might actually get better fuel economy in your next lowest gear at 40 mph. Quirky, but worth investigating IMO.
another possibility is that he shifts at about that point.