[gone-ot]

...it's all about "loading" on the engine.

...switching to a lower gear allows the engine to rev higher which reduces its "load" and is shown by the higher vacuum reading, because the vacuum gauge closely parallels the engines "loading".

...best example, is the older non-smog engines, where coasting down a hill in high gear would almost "peg-out" the vacuum gauge...an almost perfect example of "no load."

...conversely, "flooring" the gas pedal would likewise "zero" the vacuum gauge...an almost perfect example of "maximum load."

...the answer to your question is to "mentally optimize" the two by aiming (throttling/shifting?) for highest vacuum reading at lowest RPM that you're comfortable with driving (the people behind you might not agree, but what the heck)...and that sometimes means driving UNDER the posted speed limit.