Food for thought, BSFC contours are usually wider than they are tall. Or, in other words, even if peak BSFC happens at a higher RPM, the increase in load from a higher gear is almost always better than lower load and higher RPM. See here for an example:
As for maintaining speed, my Insight is geared such that I can basically only maintain speed in 5th gear on level ground or slight inclines. Any appreciable grade and I have to downshift, sometimes even two (or three) gears. I get my best fuel economy in whatever gear is the highest that I can barely maintain speed in.