The Union of Concerned Scientists produce a nice MPG equivalent map. Here are a relatively new one and and one a couple of years old (2015 v 2012?).
This shows in many places, that an EV is better than anything but a Prius, and it also shows, that the grid is cleaning up fast. So the EV you buy today, just keeps getting better with regards to emissions:
The improvement certainly seems like great news, but keep in mind it is mostly due to substitution of coal by natural gas. As the environmental costs of NG extraction are more clearly accounted for the CO2e value is going to rise -- quite a bit. Not as bad as coal, but not as good as it appears. I think the real advantage of NG is the reduction in SOx and Nox, not the CO2.
Second, these maps were calculated by using average grid emissions for each region. I consider that method flawed because the cleaner source fuels do not scale with increased demand. The studies performed by the US national labs use marginal emissions, which are almost exclusively fossil fuels. In marginal emissions terms it plays out like this:
NG is ~ 1 lb CO2/kWh (by current methods that do not do a good job of counting extraction costs.)
Coal is ~ 2 lb CO2/kWh
Petrol is ~ 24 lbs CO2/US-gallon WtW
Using a 3 mile/kWh EV, the cars are about equal in CO2 emissions at
72 MPG petrol car in a 100% NG fueled EV
36 MPG petrol car in a 100% Coal fueled car
54 MPG petrol car using the median fossil fuel mix in the US
Night-time charging: More coal
Day-time charging: More NG
Third,and perhaps most important from an unbiased environmental perspective, this map ignores opportunity cost. For example, a person who buys an inexpensive, efficient ICE car and plows the money saved into PV is way, way ahead in an environmental sense compared to a person who buys a Tesla and relies on the grid for energy.
I am as much pro-Tesla as anybody here, but I like to temper my enthusiasm with facts.