1995 Mazda Protege Question: Why does my mazda etude 1.8 sedan 1995 have high fuel consumption

My mazda etude 1.8 litre sedan 1995 model is very heavy on petrol. It does about 430km on a 50litre tank and I do not know how to fix this problem. I bought it recently and it has had the same consumption since. It has 244000km on the clock. I serviced it recently, changing the spark plugs, oil and air filters but it is still doing 430km on a full tank. The timing belt makes a slight squealing sound and the car sometimes jerks a bit during acceleration when I am driving on the highway. Could this be what is causing the high fuel consumption? What could be causing the jerking and what should I do to get a fuel consumption rate of at least 12km on a litre.