Two Charts Show Where Our Cars' Carbon Savings Come From

Vehicle MPG average. (Chart by Michael Sivak and Brandon Schoettle, U. Mich)

UPDATED with more recent data.

American drivers are using less gas to get around, but it's not because they're driving fewer miles. Still, we're spewing less pollution per driver according to research out of the University of Michigan.

The average fuel economy of new vehicles sold in the U.S. in January hit a record 23.0 MPG. That's up 0.8 m.p.g. from December. That also means that drivers are spewing less CO2 on average. The University of Michigan's Transportation Research Institute crunched the numbers here and found a steady decrease in CO2 emissions per driver since April 2009 and a net decrease of 14 percent since October 2007 when they began measurements.

U. Mich's Eco-Driving Index (EDI) created by Michael Sivak and Brandon Schoettle estimates the average monthly emissions generated by an individual U.S. driver. The EDI takes into account vehicle fuel economy and distance driven. Together that captures the fuel consumed and therefore CO2 emissions for internal combustion engine vehicles.
The reference point is October 2007 when the EDI started, so that's 1.00. As the chart below shows, December 2011 (the latest month available) is a 0.89, 11 percent less than the start point in October 2007.
What's interesting is that average distance driven hasn't changed all that much, so the CO2 savings are coming almost entirely from fuel economy improvements. President Obama has called for carmakers to double their fleet fuel efficiency by 2025.

Trends in fuel consumption and CO2 emissions since 2007. (Chart by Michael Sivak and Brandon Schoettle, U. Mich)

*An earlier version of this post used November's EDI data and stated a different level of reduction in CO2 emissions.