Gas mileage of new vehicles sold in the U.S. rose 0.2 mpg last month, say researchers at the University of Michigan Transportation Research Institute.

Average fuel economy (window-sticker values) of cars, , vans and SUVs purchased in October was 24.8 mpg, according to UMTRI researchers Michael Sivak and Brandon Schoettle. Gas mileage is up 4.7 overall since October 2007, the first month of monitoring.

"The most recent improvement likely reflects the net effect of two opposing trends: the improved fuel economy of model year 2014 vehicles and the decreased demand for fuel-efficient vehicles because of the recent reduction in the price of gasoline," Sivak said.

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 August, the EDI tied its best mark at 0.80 (the lower the value, the better). The index currently shows that emissions of greenhouse gases per driver of newly purchased vehicles are down 20 percent, overall, since October 2007.