The average fuel economy of new U.S. vehicles increased 0.3 mpg and logged the first improvement since August, according to the University of Michigan Transportation Research Institute's monthly report.

The average window-sticker number for new cars, trucks, SUVs, and vans reached 25.4 mpg in January. It reached 25.1 mpg in December. During the summer months, the average mpg of new vehicles purchased remained about 25.5 mpg. The monthly report is produced by Michael Sivak and Brandon Schoettle.

The fall in new-vehicle fuel economy since late summer mirrors a slide in gasoline prices that began in late June. Sales of pickup trucks, vans and SUVs surged in the second half of 2014. This trend has been seen in fleet sales as well as the retail channel.

New-vehicle fuel economy has risen 5.3 mpg since October of 2007, when the UMTRI began its tracking.

The University of Michigan's Eco-Driving Index, which tracks average monthly greenhouse gas emissions, reached 0.77 in November, a decline from the 0.80 rating in October. A lower score is better than a higher score.