Actual ford production vehicle     Liquefied Petroleum Gas (Propane) powered automobile More than 1/2 a million vehicles toady are currently using propane/liquefied petroleum gas. Propane is a gas but it does turn to a  liquid under moderate pressure. When liquid propane is drawn from the tank, it changes back to a gas before it is burned in the engine. Propane is a clean fuel which has many of the same properties as gasoline.  One gallon of propane contains less energy than a gallon of gasoline 82,485 Btu for propane, versus 115,400 Btu for gasoline. Propane-powered vehicles produce about less organic gases than gasoline vehicles. Converting an automobile or light-duty truck from gasoline to propane costs about $1,000 to $2,000 and can usually be done in one day. Most automobile manufactures actually offer vehicles from the factory already converted to propane, but are usually a special order item.