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.
Let's take our calculators out and check Dvice's numbers: Americans have 250 million cars. Supposing each of these cars could be fitted to a 25 kWh battery (the Tesla Roadster holds 53 kWh, the Chevy Volt will use a 16 kWh pack) and that we can drive 2 or 3 miles per kWh. Assuming that all these cars are used at current average levels (something the source doesn't exactly specify), this translates into 100 charge cycles per year. Total electricity bill: 600 billion kWh per year, and that's just 15 percent of current production (about 4 trillion kWh).
Now, onto the carbon figures: Every kWh from a coal plant produces two pounds of CO2, so we're talking 1.2 trillion pounds of CO2. The U.S. burned 3.3 billion barrels of gasoline in 2008, and a single gallon becomes 20 pounds of CO2 at the exhaust pipe, which turns out to be which about 3 trillion pounds of CO2. Ergo, coal emits 60 percent less.