I believe the points Duke 2.6 were making is that engine efficiency is maximized when the losses are lowest. The items he cited that contribute to losses are intake manifold vacuum and internal friction. Lower rpm operation reduces internal friction, and low intake manifold vacuum reduces the work the engine has to do to get air in the cylinders (called pumping losses - the intake cycle is one part of this loss mechanism, and the exhaust cycle is the other part).

The economy indicator needle on the dash is a vacuum gage that measures the intake manifold vacuum and presents the reading on scale of relative operating economy. That vacuum is created by the engine sucking air past the intake throttle plate. Intake vaccuum is a very helpful parameter to measure to understand how well the engine is running or tuned. Unfortunately, the indicator for "economy" in the car is very simplistic, and merely intended to be a visual aid to let people, who can't make the connection between the throttle position with their right foot and economy, know when they are deep into the pedal they are likely using more fuel than they need to use. The gage has no common sense and only measures vacuum, so it cannot consider the actual driving conditions (are you accelerating, going up a hill, have a heavy load, etc). It should be used as an "indicator" and not a gage that measures actual economy of operation.

Making the vacuum takes energy away from the rear wheels, so you have to burn more fuel which reduces your mileage. As odd as it may seem it is not trivial. The object of low resistance, high flow air cleaners is to reduce this effect once the throttle is open fairly far, and they are often cited as being capable of making significant (measureable) fuel economy gains. Technically the engine operates with the fewest pumping losses with a throttle position open as far as it will go. So, better operating efficiency is achieved with the throttle as far open as it will go without triggering any fuel enrichment features to achieve maximum power, such as are used when you are at wide open throttle (WOT).

Consequently with an automatic transmission, to achieve the highest operating efficiency (best fuel economy) the manufacturer programs the transmission shift points to occur at an rpm with lower friction losses (a lower rpm than where you achieve full power) and where you will accelerate between shifts with the throttle open to near the threshold of turning on the fuel enrichment feature needed for delivering high power.

For most of the automobiles we drive, at steady state, flat and level road conditions at speeds to keep up with all the speeding traffic of today, you will be able to drive in high gear and have your economy needle pegged on the "economy" side of the indicator, or at least out of the red zone. If this is not the case, you may have an engine tuning parameter that is not correct. I hope this helps. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)