moisture you see coming out of the tail pipe is steam, one of the by products of combustion. gasoline is HC or hydrocarbon + O2= CO2 and H2O. there are five gases that come out of your tail pipe: carbon monoxide (CO), carbon dioxide (CO2), unburnt fuel (HC), NOx (oxides of nitrogen as a result of high combustion temperatures) and oxygen (O2). the steam is the most visible byproduct.