OK I'm feeling kind of dumb - let's say a car gets 20mpg - if we are emitting 411 grams of CO2 per mile that comes out to 8,220 grams of CO2 in that 20 miles, which is 18.1 pounds of CO2. How do we get 18.1 pounds of CO2 out of a gallon of gas that weighs 6 pounds?
Not a stupid question, it's actually a common one.
The answer is that only the C in CO2 comes from gas, the O2 comes from the air.