Researchers at the University of Michigan Transportation Research Institute say gas mileage of new vehicles sold in the U.S. improved in January, while emissions are now at their best mark ever.

According to researchers Michael Sivak and Brandon Schoettle, average fuel economy (window-sticker values) of cars, light trucks, vans and SUVs purchased last month was 24.9 mpg, up 0.1 mpg from December and up 4.8 mpg since October 2007, the first month of monitoring.

In addition to average fuel economy, Sivak and Schoettle issued a 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 the fuel used per distance driven and the amount of driving - the latter relying on data that are published with a two-month lag.

The index was at an all-time low of 0.79 during November, which indicates that emissions of greenhouse gases per driver of newly purchased vehicles are down 21 percent, overall, since October 2007.