If you open the throttle half way at low revs - say 2000 - manifold vacuum goes to essentially zero so pumping loss is minimized, but you do not activate WOT enrichment, and the engine will not downshift, so internal engine friction remains low. This is the most efficient acceleration mode, but acceleration will be less than WOT or enough throttle to cause a downshift.

Another way to understand engine efficiency is gearing. Say your engine is geared for 3000 at 60 and achieves a certain fuel economy on a level road. If we regear it for 2000 at 60, it will have to produce the same power to cruise on a level road. This means the throttle will have to be opened more which will reduce pumping loss because of reduced vacuum, and the engine would not be dissipating as much energy as internal friction. The result is that the engine is operating at higher thermal efficiency and fuel consumption will drop about 10 percent.

The downside is that the engine will have less potential power at the lower revs, so hill climbing may require a downshift.

One reason why modern cars achieve better fuel economy than older cars is that the modern cars typically have more transmission ratios and taller cruising gears. The Corvette is a good example. Despite a 400 HP engine its sixth gear yields about 1500 at 60, and it achieves 28 MPG on the EPA highway cycle.

BTW, MB removed the "economy gage" because it was misunderstood by owners. Using manifold vacuum as a surrogate for economy is ONLY valid for level road cruising, not acceleration or hill climbing.

If you cruise at progressively higher speeds on a level road, you will notice that as cruise speed is increased manifold vacuum is less and the needle approaches the red zone. The power to maintain speed increases with the CUBE of speed, and most people understand intuitively that higher cruise speed reduces fuel economy.

Duke