ANN ARBOR—After two months of increases, the average fuel economy of all new vehicles sold in the United States fell by a half mile per gallon last month, say researchers at the University of Michigan Transportation Research Institute.

Average fuel economy of cars, light trucks, minivans and SUVs purchased in December 2011 was 22.2 mpg, down from 22.7 mpg in November and 22.6 in October.

According to Michael Sivak, research professor and head of UMTRI’s Human Factors Group, average fuel economy of all new vehicles bought last month was down from 22.3 in December 2010, but up slightly from 22.0 in December 2009.

The all-time high of 23 mpg occurred in March 2011. Average fuel economy for new vehicles sold is now up about two miles per gallon from just four years ago.

In addition to average fuel economy, Sivak and UMTRI colleague Brandon Schoettle issued their 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 vehicle fuel economy and distance driven—the latter relying on data that are published with a two-month lag.

During October 2011, the EDI stood at 0.85, down from 0.87 in September and 0.86 in August. The index currently shows that emissions of greenhouse gases per driver of newly purchased vehicles are down 15 percent since October 2007.

Related Links:

• Fuel economy calculations, along with a graph and table of current and recent MPGs
• Eco-Driving Index calculations, along with a graph and table of current and recent values