According to a recent article in the WSJ (see link below), we are witnessing a phenomenon in the energy market of crude oil and natural gas. It states that the price for natural gas has fallen 79% from its high in the summer of 2008 and that gas has fallen significantly less, thus, creating a historic price gap between the two energy sources where the price of oil is nearly 37 times the price of natural gas.
Because oil and gas fulfill the same fundamental purpose, that of an energy source, consumers and business will begin changing behavior if the price gap continues over the long-run. In the short-run, the upfront costs of switching from oil to natural gas might be too expensive to justify, but in the long-run consumers and businesses will begin replacing worn-out equipment with a natural gas equivalents, worn-out automobiles with natural gas equivalents, and other worn-out durable goods with natural gas equivalents. As these businesses and consumers makes such changes, the demand curve for natural gas will shift out (right) representing their increased demand for natural gas at all prices, and the demand curve for oil will shift in (left) representing their decreased demand for oil at all prices. As the two markets adjust to the change in demand, eventually their prices will drift back to the historic equilibrium of the price of oil being "6 to 12 times more...than natural gas."
I know you will all be watching closely to see if and when it becomes economically reasonable to convert your cars and trucks to operate on natural gas.