The average fuel economy of new vehicles sold in the U.S. in December fell to 25 mpg — down 0.2 mpg from a revised November value, according to Brandon Schoettle and Michael Sivak, researchers from the University of Michigan's Transportation Research Institute (UMTRI).

The value for December is up 4.9 mpg since October of 2007 (the first month of monitoring), but down 0.5 mpg from the peak of 25.5 mpg reached in August of 2014.

For 2017, the average fuel economy of vehicles sold was 25.2 mpg, which was unchanged from 2016.

The University of Michigan Eco-Driving Index (EDI) — an index that estimates the average monthly emissions of greenhouse gases generated by an individual U.S. driver — was 0.83 in October, which was unchanged from September (the lower the value, the better).

The EDI indicates that the average new-vehicle driver produced 17% lower emissions in October than in October of 2007, but 5% higher emissions than the record low reached in November of 2013.