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Smart answer - A vacuum gauge is an instrument used to define a numeric value to the negative pressure in an enclosure or duct.

The answer you're looking for - A fairly primitive way to gauge fuel economy by gauging manifold vacuum, or in this case, port vacuum which can be used to roughly define engine load. This data then enables the operator to make changes based on the information given, such as change gears increase or decrease throttle to maintain desirable load, which translates to better fuel economy.