Chemical Reaction
If bleach is poured into a gas tank, presumably with gas in it, the bleach itself will not harm the car. However, the chemical reaction between the chlorine in the bleach and the gasoline in the tank will definitely create havoc with your vehicle's fuel system and motor. The chlorine will react with the organic compounds from which gasoline is formed (remember, gas is refined oil and oil is an organic compound), creating an abnormal amount of rapid oxidation (rust). This reaction would, in effect, break down the metal in the gas tank dozens of years faster than normal. In fact, the adverse affects would become painfully obvious within two to four days.

Effects
If this reaction takes place and the car is not driven, repairing the damage may only involve the replacement of the damaged fuel tank and flushing the fuel system. However, if the car is driven, a mixture of bleach and fuel will be distributed throughout the fuel system, corroding all metal, rubber and plastic it comes into contact with. In addition, a high volume of rust sediment will be forced through the fuel injectors and into the engine. In the best cases, the fuel injectors will clog and no gas will get through to the engine. In the worst cases, some bleach-filled gasoline will enter the engine and affect enough of the engine to seize up the engine block, rendering it useless.