Looking at the manifold pressure on the scanner, or simply hooking up a vacuum gauge, will help you find the “island of best fuel economy” in the sea of engine performance.

Notice that in nearly all BSFC graphs the best spot is at low vacuum but at an engine speed where the pistons are moving less than 1500 feet per minute.

Another way of saying this is:

Gasoline engines get best fuel economy when
the engine is making high torque
without scraping the piston rings against bore walls too fast.