Wanted to post another clarifying thought about the vacuum theory.
As Sean mentioned, a throttle blade is a restriction. When it comes to efficiency it would be best to be at WOT, this is one of the reasons diesel engines get such good mileage. But, for a gas engine, more throttle equals higher cylinder pressure (due to more air flow) which causes a higher demand for fuel... So although the engine is less efficient with the throttle closed, there is less fuel consumption, and that's primarily what we're concerned with here.
So how do we measure fuel consumption without doing it directly? We look at the vacuum guage, the higher the vacuum reading, the less the throttle is open and therefore the less fuel is being metered with the air. Because the vacuum guage is actually reading the restriction, the negative pressure which relates consequently to lower cylinder pressures and once again, less fuel consumption necessary to match the air being sucked in.
So, if the vacuum guage does not read the highest number possible while cruising, this means that the throttle is open more feeding the engine more air, and yes more fuel, to maintain speed. There is a greater need at lower rpms, (where the engine is making less power) for increased cylinder pressures to maintain speed.
Was that good, did that all make sense? I think I've said most of this before, just trying to be more clear.