Not 20 lbs of CO2 per mile. Most of the mass of gasoline is carbon. But most of the resultant CO2 mass is oxygen. So each gallon (6 lbs) of gasoline produces about 20 pounds of CO2. So in your example of a 20mpg car, each mile is a pound of CO2.

Its 23 pounds of CO2 per 7 pound gallon of gasoline. Simple chemistry. A (CH2)x unit in gasoline combusts to CO2. Thats an increase from 14 atomic units to 44 atomic units by replacing H by O.

Since the average internal combustion vehicle uses 500 gallons a year, that is about 6 tons of CO2.

C weighs 12

O weighs 16

And there's two Os in a molecule of CO2, and gas is mostly a bunch of Cs. The Os come from the air.

(and it's 20 pounds per gallon, not mile)

This is a great question. I had exactly the same thought some time ago and was scratching my head where the extra mass was coming from.

Short answer, it pulls oxygen from the air to make the CO2. Most of the mass of CO2 is from the O2 that came from the air, not the tank.

Combustion is a reaction between fuel and oxygen. Part of the weight comes from the fuel, the other part comes from oxygen in the atmosphere.