An unexpected drop in vehicle fuel economy

Here's a surprising auto-related statistic for September: The average fuel economy of new vehicles sold in the U.S. last month dropped significantly, declining to 25.3 mpg -- a 0.5 mpg drop from August.

That data was complied via the University of Michigan's Eco-Driving Index (EDI), which for the past seven years has been measuring the average monthly emissions of greenhouse gases generated by an individual driver in the U.S. The EDI takes into account both vehicle fuel economy and the distances driven.

Michael Sivak, director of Sustainable World Transportation at the university's Transportation Research Institute, says two components are behind the September fuel economy decline.

"The first factor," he told CBS Moneywatch, "is the increased share of light trucks and SUVs among the light-duty vehicles sold. The second factor is the continuing decrease in the price of gasoline, resulting in a temporary reduction in the demand for more fuel-efficient vehicles of any kind."

According to the EDI, average fuel economy has risen by a substantial 5.2 mpg since the group first began collecting data. "During the course of our monitoring, vehicle fuel economy has improved by 26 percent," said Sivak, "from 20.1 mpg in October 2007 to 25.3 mpg in September 2014."

And even though light trucks and SUVs have lower overall fuel economy than cars, "all types of vehicles have seen sizable improvements during the past seven years," he added. "I expect this trend to continue, motivated partly by the CAFE [Corporate Average Fuel Economy] standards and partly by consumer demand."

Even with the strict fuel efficiency standards announced by the Obama adminstration two years ago, the International Energy Agency says the U.S. still lags behind some European and Asian countries on its global Fuel Economy Readiness Index.