In 2004, 45% of automotive CO2 emission came from the US -- 5% of world population

The United States has 5% of the world’s population and 30% of the world’s automobiles, but it contributes 45% of the world’s automotive CO2 emissions.

In 2004, U.S. cars and light trucks emitted 314 million metric tons of carbon-equivalent (MMTc). That equals the amount of carbon in a coal train 50,000 miles long—enough to stretch 17 times between New York and San Francisco. In fact, the amount of CO2 emitted from oil used for transportation in the United States is similar to the amount from coal used to generate electricity.

http://www.edf.org/sites/default/files/5301_Globalwarmingontheroad_0.pdf