The standard caveats apply, such as the unpredictability of the type of fuel used, but this index could indicate if the fuel economy of new cars is improving or not. According to the study, emissions are indeed decreasing, with a 14% reduction of emissions per driver of newly purchased vehicles between October of 2007 and April of 2011.
The index was developed by Michael Sivak and Brandon Schoettle of the University of Michigan's Transportation Research Institute.
Itir Sonuparlak of The City Fix comments, "Although the market's current vehicles have better fuel-efficiency than that of previous vehicles, we are a long way from cutting our carbon emissions in a meaningful and effective way."
Thanks to Jeffrey Riecke