Fuel tanks don't catch fire if you shoot at them with tracer bullets (see the old episode of mythbusters), why should they explode when the car hit a tree? It was a nice car, well designed, and surely the engineers at BMW had considered various crash situations, including a front colliosion with a blunt object.
People mentioning mythbusters as if it is actually a relevant data point is really annoying.
They are an entertainment show, nothing more. Because they shot a tracer bullet at a gas tank (really that's the "evidence"?) means exactly nothing in the real world.
