Calorific Value of a fuel is the amount of energy produced by a unit mass of the fuel on its combustion.
Natural Gas is at the top of the table at 13Kcal/g followed by Propane and Butane at 11.9 and 11.8 Kcal/g, respectively. And Gasoline ranks fourth at 11.3Kcal/g.
Clearly Natural Gas is one of the most efficient source of energy which has not been utilised to it’s full potential!