The National Journal reports:
That’s because the government’s calculations have changed since 2008, and cars manufactured before that have been downgraded about 2 to 3 miles per gallon from their original advertised fuel economy.
“The older test was biased a little high,” said Doug Greenhaus, chief counsel for environment, health, and safety at the National Automobile Dealers Association. “In almost every instance, [the new test] is a lowering.”
The new calculations add factors such as acceleration, air-conditioner use, and outside temperatures.