From http://mythbustersresults.com/crash-and-burn
To begin, Jamie and Adam brought a car to the top of a quarry. The car accidentally rolled off the cliff before the accelerator was pushed down with a weight. The car tumbled only part of the way down the quarry walls and did not explode. The MythBusters next brought in a fresh car and designed an improved launching method. The second car sped of the cliff and crashed much more impressively but still did not explode. Investigation of the wreckage showed that the gas tank was in a very well-protected position and barely damaged. At this point, the myth was busted. To find out if an exposed gas tank will even explode under impact, Jamie and Adam used a large, falling weight to crush a gas tank and added boxes of matches to simulate sparks. The high speed footage showed a wide spray of gasoline mist that quickly ignited into a large fireball. To attempt to recreate an explosion with an actual falling car, Jamie and Adam attached a gas tank externally to the front of a car and covered it with boxes of strike-anywhere matches. Upon crashing, the fuel tank did explode, but the result was less than hoped for because Jamie forgot to release the parking brake during launch. Finally, to duplicate the car explosions seen in action movies, Adam and Jamie successfully used a ramp, plastic jugs of gasoline, detonation cord, and a wireless trigger.