The new Ford Fusion Energi plug-in hybrid boasts 108 MPGe (miles per gallon of gas equivalent).

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The efficiency of a car engine can be defined in terms of a miles-per-gallon equivalent.

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Thirteen teams are competing to build a plane that can fly at least 100 miles per hour and achieve the equivalent energy efficiency of 200 miles per gallon of fuel on a 200-mile flight.

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The Spark EV also gets the equivalent of 119 miles per gallon in testing monitored by the U.S. Environmental Protection Agency.

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That allows it to get the equivalent of 54 miles per gallon.

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When advertising a new car's efficiency, for example, it is more convincing to talk about the number of extra miles per gallon it does, rather than the equivalent percentage fall in fuel consumption.

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Under new rules established by the Obama Administration, carmakers must achieve the equivalent of 35.5 miles per gallon, on average, by 2016.

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Under the proposal, model year 2017-2025 cars and light-duty trucks are expected to achieve increases in fuel efficiency equivalent to 54.5 miles per gallon by 2025.

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