Gallon of Gas Plus Air Makes for Plenty of Pollution

Our local paper runs the Car Talk column - this one was really great. I knew those car talk guys were smart - but they could be chemistry teachers!

Dear Tom and Ray:

I've heard that each gallon of gas burned by a car releases about 20 pounds of CO2 into the atmosphere.
I'm not a chemist, but even allowing for some creative reactions and recombinations going on, how does combusting about six and a half pounds of fuel turn into 20 pounds of CO2? I can't quite get the math
to add up. What am I missing? -- Mike

Click here for the answer.