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Basically, gasoline burning in air is an exothermic chemical reaction (releases heat). The gasoline molecules and oxygen molecules react in certain proportions- one fuel molecule will combine with around 14 or so air molecules. You can't burn any more fuel without adding more air. Fuel and air mixed in the chemicaly correct ratio would be the perfect reaction...no waste. But that never happens in car engines b/c it builds up a lot of heat. So engines are deliberatley tuned to put in extra fuel to cool things down. Air/Fuel readings like 12:1 mean that every fuel molecule is being mixed with only 12 air molecules, so not all the fuel will burn. The Air/Fuel ratio your car runs depends on the state of tune and types of mods, but there is an ideal range. When the number is lower (like 9:1, say) that's called running rich. Way too much fuel, actually inhibits combustion (loses power) and causes black smoke (and hydrocarbon emissions). The closer it is to the stoichiometric ratio (around 14:1), the leaner you are getting, and the hotter your combustion chamber is getting, increasing chances for detonation and piston melting (and increasing NOx emissions).
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