Brother Ihsan,,,dont go for catalytic converter removal as it will simply make your car sound very irritating,,,I have also a 1300 cc belta 2007 model and getting average of 11 to 12 on AC and 13 to 14 without AC,,,Had you tried hi octane any time if not then try to use it 4 to 5 times mixed with normal petrol,,,Get your car plugs changed and also try using original Toyota oil and air filters.,,,hope these will improve avearage and finally keep in mind car avearge is under your foot,,,the way we drive our car also matters much in its fuel economy.