How does gas burned by a car turn into so much CO2? Tom and Ray explain.

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

Tom: You're missing the oxygen, Mike. The ideal air-to-fuel ratio in a car is 14.7-to-1. So to combust a pound of fuel, you need to combine it with almost 15 pounds of air.

Ray: That means in order to combust a gallon of gasoline -- which weighs about six and a half pounds -- you'd combine it with almost 100 pounds of air! So, you can see quickly how you can end up with 20 pounds of a single byproduct.

Tom: During combustion, a bunch of the carbon molecules (C) in the gasoline attach to two oxygen molecules (O) from the air, and come out the tailpipe as CO2. And CO2 -- one carbon and two oxygen molecules -- weighs about three times as much as the carbon molecule you started with.

Ray: Burning gasoline is a complex equation. You end up with all kinds of byproducts, including CO, H2O, HBO, Cinemax, Flomax, etc. But the way you get 20 pounds of CO2 from six and a half pounds of gas is by combining the gas with a heap of oxygen during the combustion process.

(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk Web site.)