Everytime someone asks about fuel economy it never fails this is brought up every time. And every time people refuse to acknowledge simple thermodynamic/fluids laws.

Fact the area of a pressure/volume graph = the work used to move the fluid. This is what we call pumping losses... The more pressure (vacuum in this case) and the more volume (displacement) the more work required. Also the lower the RPM the less dynamic mechanical friction you have.

So it's pretty much opposite of what everyones been saying, if you want greatest gas mileage you want to 'lug' your engine at a low RPM (low friction and flow rate) with the throttle blades open as far as you can (low vacuum). This is exactly why all the new motors use active cylinder shut-down in combination with drive by wire.

As far as why the old vacuum gauges listed 'economy' in the high vacuum range? Simple, high vacuum also means you're not accelerating hard .