The typical fuel economy of vehicles sold in the United States showed no change in September, according to researchers at the University of Michigan Transportation Research Institute. However, the sales-weighted model year average for 2017 was up slightly.

Vehicles sold in the U.S. during September had an average window sticker combined fuel economy rating of 25.3 miles per gallon. This was the same value recorded in August, but up from a rating of 25 miles per gallon in September 2016.

The sales-weighted fuel economy figures are determined by UMTRI researchers Michael Sivak and Brandon Schoettle, who have been tracking the data since October 2007. In that month, the average new vehicle purchased in the U.S. got 20.1 miles per gallon. The reports are based on monthly sales of cars, light trucks, SUVs, and vans as well as ratings published in the Environmental Protection Agency's "Fuel Economy Guide."

The highest average sales-weighted fuel economy figure was recorded in August 2014, when the typical newly purchased vehicle had a rating of 25.5 miles per gallon. Following a plunge in gas prices and increased sales of trucks and SUVs, each month since the record high has had an average between 24.8 miles per gallon and 25.4 miles per gallon.

However, the model year average showed its first improvement in four years. This average is based on fuel economy data between October and September, and rose from 20.8 miles per gallon for 2008 to 25.1 miles per gallon for 2014. Although the model year average remained flat for 2015 and 2016, it inched up to 25.2 miles per gallon for 2017.

Sivak and Schoettle also update a figure called the Eco-Driving Index, which estimates changes in greenhouse gas emissions generated by individual drivers of new vehicles. These emissions levels are compared to an October 2007 baseline. The index is updated on a two-month delay due to a corresponding lag in travel data from the Federal Highway Administration.

The Eco-Driving Index for July was 0.81, indicating that emissions produced by new vehicle drivers were 19 percent lower than in October 2007. This was the lowest reading since July 2016, but remained higher than the record low of 0.78 set in November 2013.

The vehicle miles traveled by drivers in July were an estimated 1 percent higher than in October 2007. The fuel used per distance driven was estimated to be 20 percent lower than this baseline.