According to HowStuffWorks, pouring sand into a gas tank has the potential to clog up the fuel filter in a car, which causes the car to become disabled. However, the amount of sand that needs to be put into the gas tank to achieve this is great, and it is not guaranteed to work.
There are old rumors that suggest that pouring sand or sugar into a car is the best way to disable it. However, pouring sugar or sand in a gas tank is not much different than pouring any other substance that shouldn't be in a gas tank. These substances are not good for the car, but this does not mean that they always disable the car.
The rumors used to state that sugar turns gasoline into a semi-solid that clogs up the engine. In actuality, sugar does not dissolve in gasoline and works much in the same way as pouring sand into the gas tank.
One of the quickest ways to disable a car is to put water into the gas tank. This is because gasoline floats on top of water. Pouring in a few cups of water causes the water to sink to the bottom of the fuel tank, and the fuel pump circulates the water into the fuel lines, causing major problems for the car.