Vehicles sold in the US since October 2007 have saved a cumulative total of about 6.1 billion gallons of fuel—equivalent to the current total consumption of all vehicles in the US for about 13 days—due to their improved fuel economy, according to a brief report by University of Michigan Transportation Research Institute (UMTRI) researchers Michael Sivak and Brandon Schoettle.
This reduction in the amount of fuel translates to a reduction of about 120 billion pounds of carbon-dioxide emissions.
Sivak and Schoettle have issued monthly reports on the the average, sales- weighted fuel economy of all light-duty vehicles (cars, pickup trucks, vans, and SUVs) sold in the US (e.g., earlier post.) The results indicate that, from October 2007 to September 2012, the average fuel economy has improved by 18%, from 20.1 mpg to 23.8 mpg.
In terms of the current savings, for the most recent month—September 2012—the savings amount to 293 million gallons of fuel, or about 5.7 billion pounds of carbon dioxide. These savings are equivalent to about 2.9% of the average monthly consumption of fuel and of carbon-dioxide emissions from all light-duty vehicles on the road.