All the talk of manifold vacuum and how it increases fuel consumption relates back to throttle position.

The more you have the throttle open the more fuel the engine requires. The RPM will relate but not as directly, as you will find to hold an engine at 5000rpm requires more throttle than to hold an engine at 2000 rpm.

The second thing that will effect your fuel economy is the speed you travel at. The fast you go the more resistance the air produces against you moving through it.

Air resistance works like this
F= 1/2 CApv^2

F=force of air resistance
C=car's drag cooefficient
A=cross sectional front area of car
p=density of air (effect by temperature and pressure)[PV=nRT]
v=Car's velocity

So you can see that the air resistance increases with the square of velocity.

That's why cars have a top speed....pushing through air is hard work.

So the slower you drive the less force it takes.

And with the manifold pressure idea - you select a gear which allows you to maintain speed with the minimum throttle position you will use the least amount of fuel.

If you have to accelerate more in 5th to keep the car going, than in 4th or 3rd...you are probably using more fuel.

Hope some of that helps - i worked a lot of this out when making a 'street dyno' - I could calculate hp or torque from just an RPM and time data log.