The average fuel economy of new cars sold in September dropped 0.6 mpg (25.2 mpg) compared to August 2014 (25.8), according to a follow-on study from the University of Michigan's Transportation Research Institute (UMTRI).

The study, produced by UMTRI researchers Michael Sivak and Brandon Schoettle,monitors monthly vehicle fuel economy and emissions, starting from Oct. 2007. The drop in vehicle fuel economy reciprocates the decline in gas prices in September and the growth in sales of pickup trucks, SUVs and crossovers, according to Sivak.

The University of Michigan Eco-Driving Index (EDI)—is an index that estimates the average monthly emissions of greenhouse gases (GHG) generated by an individual driver. The EDI takes into account both vehicle fuel economy and distance driven (the latter relying on data that are published with a two-month lag).

The EDI was last updated Sept.30, 2015 and data shows that the EDI for July was 0.82, showing that there was a reduction of approximately 18 percent in emissions per driver of newly purchased vehicles versus an EDI of 1.00 for October 2007.