Now that all the specs are out, we can compute an AutoEcoRating for the Chevy Volt. The result? An eRating of 161, which makes it 40% greener than the average new car or light truck. The average new vehicle sold last year rates a 95 on our scale based on a lifecycle analysis, counting pollution from both the taipipe and the power plant. Because eRatings are in inverse proportion to total pollution, that difference translates to a 40% advantage for the Volt.
How did we come up with that number, you might ask? As General Motors points out, the more one drives electric, the greater the efficiency. Based on the national averages for emissions from power plants, refineries, auto factories and the other pollution sources tallied into AutoEcoRatings, that efficiency does translate to lower pollution per mile.
In fact, if you drive the Volt on battery power only, its eRating would jump to 183. On the other hand, if you ran the battery down and never plugged it in again (now why would anyone do that?), its eRating would drop to 138. That’s still a lot better than the average vehicle, of course, given that the Volt gets pretty good fuel economy even when running on gasoline alone. (We’ll compare it to regular hybrids such as the Toyota Prius in a future post).
For computing the eRating, the key assumption is how much plugged-in driving will result given the Volt’s design, which has an EPA-estimated all-electric driving range of 35 miles. To answer that question, we drew on a government study that analyzed American driving habits to estimate the electric vs. gasoline shares of driving for a plug-in hybrid having a given electric range. That study implies that 58% of total driving would be in electric mode.
Of course, it seems likely that many Volt drivers will try to maximize the amount of time they drive petroleum free, and only time (and lots of survey data) will tell what the actual average turns out to be.
 Elgowainy, A., A. Burnham, M. Wang, J. Molburg and A. Rousseau. 2009. Well-to-Wheels Energy Use and Greenhouse Gas Emissions Analysis of Plug-in Hybrid Electric Vehicles. Report ANL/ESD/09-2. Argonne, IL: Argonne National Laboratory, Center for Transportation Research. February.