Up 1.3 MPG From 2011, and Up 2.9 MPG From 2008The Transportation Research Institute at the University of Michigan has been keeping track of the average fuel economy numbers of new vehicles sold in the U.S. since late 2007 (you can see the data below, the first full year is 2008). The numbers for December have been crunched, and it turns out that 2012's average was 23.8 MPG, an increase of 1.3 MPG over 2011, and 2.9 MPG over 2008.
Another thing is that since the economic crisis of 2008, fewer new vehicles have been sold in the U.S., so they are more fuel efficient and there's fewer of them. This probably means that many people are keeping their old vehicles longer, but also that people are finding other ways to get around (walking, cycling, transit...). These delayed sales might seem like a bad thing (older, less fuel-efficient, and dirtier models stay on the road longer), but if we take the long view, they are probably a good thing; by the time people decide to buy, there will be more fuel-efficient models on the market to pick from, and new CAFE fuel economy rules will have had more time to take effect.
Interestingly, the UMTRI writes:
The University of Michigan Eco-Driving Index (EDI)--an index that estimates the average monthly emissions of greenhouse gases generated by an individual U.S. driver--stood at 0.79 in October. This value indicates a record improvement of 21% since October 2007, and ties the revised value for September. 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).