Only if it's crushed with enough force that the gas spraying out forms a combustible mist. A merely leaking tank doesn't get enough oxygen quick enough to explode. It just burns. They test this and also crash several cars down and fail to achieve the effect. For one they even mount the gas tank on the front bumper and tape boxes with matches on for sparks. Still no explosion when the car falls down and hits the ground tank first, only burning. They don't completely rule out that it might happen under very special circumstances, but it would be very unlikely.