… if we
today were driving cars of the same size and power that were
typical in 1980, concludes a new study by MIT economist Christopher
Knittel. Of course, the Obama administration has set new corporate
average fuel economy standards at 35.5 mpg for 2016. MIT
News summarizes
Knittel’s findings:

Contrary to common perception, the major automakers have
produced large increases in fuel efficiency through better
technology in recent decades. There’s just one catch: All those
advances have barely increased the mileage per gallon that autos
actually achieve on the road.

So what happened to the missing gas mileage?

…between 1980 and 2006, the average gas mileage of vehicles
sold in the United States increased by slightly more than 15
percent — a relatively modest improvement. But during that time,
Knittel has found, the average curb weight of those vehicles
increased 26 percent, while their horsepower rose 107 percent. All
factors being equal, fuel economy actually increased by 60 percent
between 1980 and 2006, as Knittel shows in a new research paper,
“Automobiles on Steroids,” just published in the American
Economic Review (download
PDF).

Thus if Americans today were driving cars of the same size and
power that were typical in 1980, the country’s fleet of autos would
have jumped from an average of about 23 miles per gallon (mpg) to
roughly 37 mpg, well above the current average of around 27 mpg.
Instead, Knittel says, “Most of that technological progress has
gone into [compensating for] weight and horsepower.”

This seems an example of the
energy rebound effect in which increased energy efficiency
encourages people to use even more energy; in this case to fuel
bigger and peppier cars.