James, You're not even close. Here's a simplified formula for the combustion of gasoline (assuming that octane is the 'average' organic component - not bad):

C8H18 + 25/2-O2 --> 8CO2 + 9H2O

On a molecular weight basis, one lb of gasoline then consumes about 3.5 lbs of oxygen, producing 3.1 lbs of carbon dioxide and 1.4 lbs of water. On a volume basis, curiously enough, one gallon of gas would produce about one gallon of liquid water. But in reality, the water would be in vapor/steam form after all exhaust components warm up.

~Gerald C (now blowing the dust off my Chem-Eng textbooks)