cylinders are always measured in liters, then the kgs of the gas depends upon pressure, e.g as a standard 55L cylinder carries 7kg gas at a pressure of 200 bars (standard pressure for CNG vehicle tanks), if the pressure is above or below, same would be the effect on kilograms of gas. therefore for CNG cars the milage is judged on Rs./kilometer not per km/kg. For petrol, the mileage is noted as per km/ltr.