A question that has been endlessly debated in internet forums is:
should the gasoline engine throttle be mostly closed (high vacuum)
or mostly open ( low vacuum)
for best fuel economy?

The (complicated) answer is:
there should be no throttle and the intake valve movements should adjust cylinder filling to suit load.

It should be more discussed that:
a particular type of engine used to have no throttle (diesel)
and thanks to the EPA
most new diesels now have throttles
and diesel fuel economy has gone down
putting more CO2 out into the atmosphere
( and slightly less NOx)