Vehicle fuel economy down, emissions up

Gas mileage of new vehicles sold in the U.S. slipped last month, according to researchers at the University of Michigan Transportation Research Institute.

Average fuel economy (window-sticker values) of cars, light trucks, vans and SUVs purchased in June was 25.4 mpg, down from 25.5 mpg in May.

"The decline likely reflects the increased sales of light trucks and SUVs in June," said Michael Sivak, a research professor at UMTRI.

Overall, fuel economy is up 5.3 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 April, the EDI rose to 0.85 (the lower the value, the better)—its worst mark since December 2011. However, the index currently shows emissions of greenhouse gases per driver of newly purchased vehicles are still down 15 percent, overall, since October 2007.

By comparison, both the EDI (0.78) and mpg (25.8) were at peak levels in August 2014.