The average fuel economy of new vehicles sold in the United States in 2015 was 25.3 miles per gallon, down slightly from 2014, according to the University of Michigan's Transportation Research Institute.
The calculations are based on the "window-sticker" fuel economy of new cars.
Researchers Michael Sivak and Brandon Schoettle found that the 2015 average was down just 0.1 mile per gallon from 2014. But the annual total was still up from the 20.1 mpg the two recorded in October 2007, when they began their monitoring.
Sivak and Schoettle have also developed the Eco-Driving Index, which "estimates the average monthly emissions of greenhouse gases generated by an individual U.S. driver." They said the index was 0.84 in October, up 0.01 from the month before (a lower number is better). The lowest index score was in August 2014, and October's emissions were 6 percent higher than that low mark. (The Eco-Driving Index is on a two-month lag, so October is the latest month in the study.)