One of these months, US new light-duty vehicle fuel efficiency will finally beat last August's highs, and we can all celebrate some real progress on the green-vehicle front. Unfortunately, September wasn't that month. Instead, the average fuel economy for a new light-duty vehicle sold in the US last month fell 0.1 miles per gallon from August to 25.2 mpg, according to a study from Michael Sivak at the University of Michigan's Transportation Research Institute (UMTRI).

New-vehicle fuel economy has now fallen for three straight months and is down from a high of 25.8 mpg last August. UMTRI also tracked new-vehicle emissions for July and found they were four percent higher than last August.

Some of the country's best-selling plug-ins, including the Chevrolet Volt extended-range plug-in and the Nissan Leaf battery-electric, are about to come out with upgraded versions. That's spurred a wait-and-see attitude in new-car buyers that in September helped drive green-car numbers down about four percent from a year earlier. This contributed to the lower fuel-economy numbers. Last month, green car sales fared relatively well compared to the rest of 2015, especially given that Volkswagen's diesel scandal helped cause a plunge in that automaker's September sales. Year-to-date green car sales across the US are down about 16 percent in 2015.

Additionally, UMTRI says falling gas prices were also a factor. Americans on average are paying $2.29 a gallon, which is down from $2.41 from a month ago and marks a plunge from the $3.30 folks were paying a year ago, according to AAA's Fuel Gauge Report.

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