A more useful illustration is to superimpose curves of constant power onto a BSFC map (given in the blue curves below):

Each curve represents a constant horsepower developed by the engine. If you are driving on a given road with a constant grade, speed, and ambient conditions, regardless of which gear you are in, it requires roughly the same amount of power to overcome aerodynamic drag, rolling resistance and driveline losses. Therefore, regardless of what gear you're in, you're riding anywhere along the same blue curve; exactly where you sit on that blue curve being only dependent on the gear you're in and therefore the RPM at which the engine is turning over.

If I take the example of operating on a certain speed such that the power is 20 HP, The lowest BSFC occurs when running at the gear that corresponds to about 1250 RPM. If I run at either a higher- or lower RPM from this point, my BSFC will increase. This is about the only point in the entire engine map where it would be disadvantageous to operate at an even higher gear if one were available (lower RPM) because of the worsening BSFC. Almost everywhere else, the lowest BSFC is achieved at the lowest possible RPM at a given power.