Doing a bit of research, the atomic weight of oxygen is 15.999u. The atomic weight of carbon is 12.0107u. Combining two oxygen and one carbon gives a weight of 44.0087u.
Carbon comprises 27.29% of the weight of CO2.
Therefore, is 20 pounds of CO2 are produced by burning 1 gallon of gas (approximately 6.3 lbs), 5.458 of the 6.3 lbs weight of 1 gallon of gas must be comprised solely of carbon atoms. That’s 87%.
Sniff, sniff... Something seems a little off.
Full disclosure: I am no chemist, so these calculations are presented only as a reasonableness check for the stated amount of CO2 produced per gallon of gas.
The bulk of gasoline is, it seems, molecules composed of carbon and hydrogen, and hydrogen doesn’t weigh that much, so it is possible that your calculations and the article’s statement are both roughly right. LTHe flip side is that in photosynthesis plants must use a whale of a lot of CO2 and water to produce a pound of biomass.
Octane is C_8H_18 - that would be 84%.
Gasoline is a mixture of other hydrocarbons though.
The energy content of hydrocarbon fuel is mostly tied up in the hydrogen. Energy released by burning hydrogen in oxygen versus the detonation of a military explosive, are about 27 to 1 in favor of the hydrogen for energy content on a weight basis.
The explosive detonation only appears to release more energy because of the brevity of the event—the shorter the time for application of a given amount of energy, the higher the peak power produced. Hydrogen has higher energy—an explosive produces higher power.