I get the benzina very fast

Hello ... I tell you that I have a Hyundai Accent Prime 1,5 of the year 2000 the problem I have is with the benzine the truth that I have not done a complete refinement, which if I change the spark plugs, but the preblema I think it goes beyond that because the needle tends to go down and also to climb what does not give me clarity of how much I have really of the benzine light and the light light He turns off again, I don't know then when I am occupying benzina from the reserve (which I don't know how many liters do). I travel only 5 kilometers and I see a considerable drop in the needle, and you have to keep in mind that I do not use it at high RPM or at high speeds or go large distances, and the car is supposed to be economical, and I have known cars with a larger engine that the benzine lasts much more and for more days to my 10 liters do not last me 3 days ... greetings ...