ANN ARBOR—Gas mileage of new vehicles sold in the U.S. posted its largest drop in nearly three years, say researchers at the University of Michigan Transportation Research Institute.

Average fuel economy (window-sticker values) of cars, light trucks, vans and SUVs purchased in September was 25.3 mpg, down from a record high 25.8 mpg in August. The last time fuel economy fell by 0.5 mpg was in December 2011.

“This large drop likely reflects the increased sales of light trucks and SUVs, and the reduced demand for fuel efficient vehicles of all types because of the falling gas prices,” said UMTRI research professor Michael Sivak.

Regardless, it was the eighth straight month that vehicle fuel economy topped 25 mpg and is now up 5.2 mpg from October 2007, the first full month of monitoring by Sivak and colleague Brandon Schoettle.

In addition to average fuel economy, Sivak and Schoettle issued a monthly update of their national Eco-Driving Index, which estimates the average monthly emissions generated by an individual U.S. driver. The EDI takes into account both the fuel used per distance driven and the amount of driving—the latter relying on data that are published with a two-month lag.

During July, the EDI improved to a record-low 0.77 (the lower the value, the better), down from 0.78 in June and the third month in a row that it had fallen. The index currently shows emissions of greenhouse gases per driver of newly purchased vehicles are now down 23 percent, overall, since October 2007.

Related Links:

• Fuel economy calculations, along with a graph and table of current and recent mpg: www.umich.edu/~umtriswt/EDI_sales-weighted-mpg.html
• Eco-Driving Index calculations, along with a graph and table of current and recent values: www.umich.edu/~umtriswt/EDI_values.html