[joss82]

This is not correct. A barrel of oil is 118kg of pure carbon but when burning this translate to roughly 450kg of CO2 (because carbon combines with heavier oxygen to form CO2)

This is without taking into account the CO2 that is emitted to extract the oil from the ground.

[acqq]

Yes, the simple "rule of thumb" is "the resulting CO2 is 3 times heavier than the gasoline burned." Because chemistry. H weights "almost nothing" (in the engineering approximation sense, to the first order of magnitude) in hydrocarbons, and the weight of a single CO2 molecule (made of two oxygen atoms, one carbon atom) is roughly 3 times the weight of C atom alone (again as a good enough simple approximation, it's a little more, but C alone is a little lighter than O, so all together with the starting H, everything comes to 3).

https://www.scientificamerican.com/article/experts-weight-ra...

For those who like more exact calculation: one molecule of C8H18 (octane) weights approx 8 * 12+18=114 masses of H, only that one molecule will produce when burnt 8 CO2 molecules, which weight 8 * 12+2 * 8 * 16=352, and 352/114 ~ 2.98 ~ 3