The mass on either side of a chemical reaction must balance, so I thought to myself, “If a gallon of fuel weighs 6 lbs, where is the other 13 lbs coming from?”

Well, it turns out that the additional weight is from the 21 lbs of oxygen that your car sucks from the atmosphere to burn that gallon of gas! Your car also spits out one gallon (8 lbs) of water in the process. 21 lbs of oxygen fills about four phone booths (~250 cu. ft.), and 19 lbs of carbon dioxide fills about two and a half phone booths (~150 cu. ft.). If your fuel economy is 25 mpg, then the above quantities occur every 25 miles you drive.

Here’s a simple mass equation:
C8H18 + (25/2) O2 = (8) CO2 + (9) H2O
hydrocarbon + oxygen = carbon dioxide + water

Basically, each Carbon atom broken from a fuel molecule grabs two Oxygen atoms from the air, and every two Hydrogen atoms grab one Oxygen atom from the air. Breaking these bonds yields heat energy.

Is this surprising to anyone else?
Disclaimer: As this not my area of expertise, I make absolutely no guarantees regarding the accuracy of this very simple example. The real-world process of combustion involves many variables.