The average fuel economy among new vehicles sold in the United States fell for the third month in a row in September, according to the University of Michigan Transportation Research Institute.

The typical vehicle sold during the month got 25.2 miles to the gallon. This average was down from 25.3 miles per gallon in August, 25.4 miles per gallon in both June and July, and 25.5 miles per gallon in May.

UMTRI researchers Michael Sivak and Brandon Schoettle have been tracking changes to the sales-weighted fuel economy of new vehicles in the U.S. since October 2007. At that point, the typical newly purchased vehicle got 20.1 miles per gallon.

The monthly average is calculated using sales figures for light vehicles such as cars, pickup trucks, SUVs, and vans, as well as the combined city and highway fuel economy of each model as determined by the Environmental Protection Agency's "Fuel Economy Guide." These ratings reflect the average window sticker fuel economy values, and they are available for about 99.7 percent of all vehicles purchased in any given month.

In August 2014, the average fuel economy among new vehicle sales reached its highest point of 25.8 miles per gallon. Since that month, each month's figure has stayed between 25 and 25.5 miles per gallon. Sivak and Schoettle credit this trend to increased sales of less efficient models such as trucks and SUVs.

The model year average, which measures the typical fuel economy for a year of sales between October and September, rose from 20.8 miles per gallon for the 2008 model year to 25.3 miles per gallon for the 2014 model year. This average has remained at 25.3 miles per gallon for both the 2015 and 2016 model years.

Sivak and Schoettle have attributed this trend to increased sales of less efficient models, such as pickup trucks and SUVs, as gas prices in the U.S. remain low. According to AAA, the average gas price in the nation has experienced year-over-year declines since mid-2014.

In addition to their tracking of fuel economy data, the researchers issue a monthly update of their "Eco-Driving Index." This figure compares the emissions generated by the driver of an average newly purchased vehicle to those produced by a new vehicle driver in October 2007. The figure is released on a two-month delay to a corresponding lag in the underlying data from the Federal Highway Administration.

The Eco-Driving Index for July stood at 0.82. This indicates that the emissions levels generated by new vehicle drivers during the month were 18 percent lower than those in October 2007. By comparison, emissions generation was 20 percent lower in July 2015 and 21 percent lower in July 2014.

Drivers in July traveled 2 percent farther than drivers in October 2007. However, they also used 20 percent less fuel per distance driven during this travel.