This article says that electric cars actually produce more CO2 emissions. Hard to know what the truth is when sources are conflicting.
A conservative and logical estimate is that Electric Cars emit more CO2 gas overall because it requires an extra conversion step.
Gasoline automobiles: Source --> Kinetic Energy
Electric automobiles: Source ---> Battery Power --> Kinetic Energy
Of course the real answer could be far more complicated since even gasoline distribution causes energy usage itself. The controversy and hardness to find a definitive answer makes it reasonable for me estimate that electric cars don't really do any obvious net change in energy efficiency OR green house gas reduction At All.
It doesn't match the hype.