I read somewhere that optimum fuel mixture is about 15/1 (15 parts air/1 part fuel). I assume that to be volume, so...about 15 gallons of air to 1 gallon of gas. How much does 15 gallons of air weigh? Not all of that is converted to CO2 (CO2 just being a waste product of the created energy), so what's left? I just don't think 15 gallons of air and 1 gallon of gas produces 28 pounds of CO2.
You are confusing your ratios. 15:1 is not volume, it is stoichiometric. For each carbon atom in gas, you have to bring in a O2 from air to burn with it. If you don’t bring in at least that much oxygen you are running too rich, cannot burn all the fuel completely and get unburnt gas and soot out the exhaust. In volume the ratio works out to more like 6500:1.
If you look through my numbers you can see how it works out to 18-20 Lbs CO2 per gallon. Personally I think the weight of CO2 is completely irrelevant relative to the total volume of atmosphere. Our atmosphere is more than 99% N2 and O2 and only a small fraction of the remainder is CO2. As long as we have enough plants and algae eager to absorb every bit of CO2 in their reach we have nothing to worry about.