I don't dispute this, but am curious to know how it happens. A gallon of gasoline weighs a bit over six pounds. It seems like a stretch to generate 20 pounds of CO2 (along with all the nitrogen oxide, all the hydrogen compounds, and the considerable weight of water) from that six pounds of gasoline.

Ah, but only the 'C' (one carbon atom) in CO2 comes from the fuel. The 'O2' (two oxygen atoms) come from the atmosphere. As a carbon atom only weighs 3/4 as much as an oxygen atom, only 3/(3+4+4) or 3/11 of the mass of CO2 produced, comes from the fuel. So 20lb of CO2 comes from 3/11 *20 or 5.45lb of carbon.

And gasoline is mostly (by weight) carbon: I'm not sure which hydrocarbons gasoline contains but let's assume it's mainly octane, C8H18. Hydrogen atoms only weigh 1/12 as much as carbon atoms, so they only make up 18/(18+12*8) or 18/114 or 3/19 of the weight of octane -- the carbon makes up the other 16/19. So 6lb of gasoline contains 16/19 * 6 or 5.05lb of carbon -- that's not quite enough to produce 20lb of CO2, but it's pretty close, and you did say a gallon was "a bit over" six pounds.

As for the rest, a similar calculation shows that the hydrogen (the other 0.95lb of the 6lb) produces nine times its own weight of water vapour (H2O, in which the oxygen atom weighs sixteen times as much as either of the hydrogen atoms) or 8.53lb of water. Nitrous oxide isn't produced from the fuel at all: the heat of the reaction causes a small amount of atmospheric nitrogen to combine with atmospheric oxygen.

Peter