If Congress passes legislation providing tax credits for buyers of natural gas fueled vehicles, what do you think will happen to all of that unused refinery capability when the demand for gasoline falls year after year?
I think this will take many years to occur - conversion to natural gas is an expensive process for automakers, and it will take a long time for the public to adapt - add to that the decreased power of gas in reciprocating engines.
Ten to 20 years is my guess.