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This is because the Department of Energy measures efficiency not in terms of miles per gallon but in terms of the reciprocal unit of gallons per mile.
The arithmetic mean of the city and highway measurements in gallons per mile corresponds precisely to the harmonic mean of the city and highway measurements in miles per gallon, giving the formula in the problem statement.
Better downforce doesn’t automatically increase fuel efficiency, though the right combination of countervailing aero efforts can help assure that it doesn’t reduce fuel economy gains made elsewhere.
Each car has two fuel economy numbers, one measuring efficiency for city driving and one for highway driving.
Most car manufacturers report CAFE values to one decimal place, and students are asked to do the same in the part (a).
It would be a useful to discuss which level of accuracy makes more sense.
This number is not the average of the city and highway economy values.
Rather, the combined fuel economy (as defined by the federal Corporate Average Fuel Economy standard) for $x$ mpg in the city and $y$ mpg on the highway, is computed as $$ \ \ = \frac.