Ann Arbor, Michigan - Gas mileage of new vehicles sold in the U.S. last dropped slightly, say researchers at the University of Michigan Transportation Research Institute.

The average fuel economy (window-sticker value) of new vehicles sold last month was 25.3 mpg—down 0.1 from July.

"This decline likely reflects the decreased price of gasoline in August, and the consequent increased sales of light trucks," said UMTRI research professor Michael Sivak.

Overall, fuel economy is down 0.5 mpg from the peak reached in August 2014, but still 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 June, the EDI rose to 0.83 (the lower the value, the better) from a mark of 0.82 in May. The index currently shows emissions of greenhouse gases per driver of newly purchased vehicles are down 17 percent, overall, since October 2007—but 5 percentage points higher than the record low reached in both August 2014 and August 2015.