After accounting for emissions during manufacturing, even if you powered an electric car with coal power only, carbon emissions of an electric car are roughly a 25-30 MPG car equivalent.
However, those numbers get much better as the electric power generation mix improves (at both the car factory and your garage). In California, now, an electric car is about equivalent to a 70 MPG car.
Note that significant efforts are underway to reduce the carbon-intensiveness of our electricity generation, so these numbers will get better over time.