"A common attack on electric vehicles is to claim that all Americans switching their cars from gasoline to electric would be counterproductive in the effort to reduce carbon emissions. The reason is that we'd simply need more polluting coal plants pumping out more carbon dioxide and get collapsed electric grids as a result. Well, we know there are emerging solutions to the grid problem but how about calculating the actual carbon numbers that result from gasoline vs. electricity from coal?
Dvice has gone and done just this, and found that, when it comes to CO2, electricity sourced from coal has a 60 percent lower impact than gasoline. Of course, this equation doesn't take in consideration other pollutants that result from burning either fuel."
http://www.autobloggreen.com/2009/07/24/study-even-with-electricity-from-coal-electric-vehilces-beat-g/