The F-Series giveth and the F-Series taketh away. When it came to January's fleetwide fuel economy for new light-duty vehicles sold, sales figures associated with Ford's world-beating truck line may have been a reason why fuel economy rose last month, even if only a little bit.

New US light-duty vehicles sold in January averaged 25.1 miles per gallon, up from 24.9 mpg in December 2015, according to Michael Sivak and Brandon Schoettle at the University of Michigan Transportation Research Institute (UMTRI). To put that in slightly broader perspective, model-year 2015 vehicles averaged 25.3 mpg, equal to the 2014 model year average and up from the 24.6 mpg average for the 2013 model year.

The better news is that new-vehicle emissions levels in November, the most recent month tracked by UMTRI, were 18 percent less than when UMTRI started tracking such figures in October 2007, though they were four percent higher than they were at the all-time low in August 2014.

As for January's fuel-economy uptick, "this increase likely reflects the month-to-month seasonal decrease in sales of pickup trucks and SUVs," said Sivak, the director of sustainable worldwide transportation at UMTRI, in a statement.

Indeed, the Ford F-Series moved 51,540 units last month, which would be great for most models but is a far cry from the 85,211 sold in December. Lots of pickups with red bows around them, we guess.

The pickup-truck theory is all the more plausible given that fleetwide fuel economy sure wasn't helped by green-car vehicle sales. Last month, sales of hybrids, plug-in vehicles and diesels plunged 23 percent from a year earlier to 25,700 units. And even factoring out Volkswagen and Audi's stop-sale on diesels, green-car sales still fell 11 percent from January 2015.

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