The fuel economy of a typical new vehicle sold in the United States at the end of 2017 fell for the second month in a row, according to researchers at the University of Michigan Transportation Research Institute.

Vehicles sold in December had an average window sticker combined fuel economy value of 25 miles per gallon. This matched the value for December 2016 and was down from 25.2 miles per gallon in November and 25.3 miles per gallon in each month between August and October.

Researchers Michael Sivak and Brandon Schoettle have been tracking changes in sales-weighted fuel economy for more than a decade. Figures are determined using monthly auto sales data as well as fuel economy information provided by the Environmental Protection Agency.

When the researchers began the study in October 2007, the average sales-weighted fuel economy among newly sold vehicles was 20.1 miles per gallon. The highest monthly value recorded occurred in August 2014, when the typical new vehicle sold in the United States had a combined fuel economy of 25.5 miles per gallon.

The average fuel economy for newly sold vehicles in 2017 was 25.2 miles per gallon, unchanged from 2016. The model year average, which measures the fuel economy average for vehicles sold between October and September, rose from 20.8 miles per gallon in the 2008 model year to 25.2 miles per gallon in the 2017 model year.

Sivak and Schoettle have also created a measure called the Eco-Driving Index, which estimates changes in emissions levels generated by the drivers of new vehicles. This figure is updated on a two-month delay due to a corresponding lag in information from the Federal Highway Administration.

In October, the Eco-Driving Index stood at 0.83. This indicates that the average driver of a newly purchased vehicle was generating 17 percent fewer emissions than a comparable driver in October 2007.

The figure was unchanged from the previous month and slightly worse than the October 2016 figure of 0.82. The Eco-Driving Index reached its lowest level of 0.78 in November 2013, indicating that the typical new vehicle driver was creating 22 percent fewer emissions than the baseline level.

The vehicle miles covered in October were 2 percent greater than October 2007 for the second month in a row. Sivak and Schoettle also found that drivers were using 19 percent less fuel per distance driven than baseline measures for the third consecutive month.