Fuel consumption problem

I am a proud owner of a Toyota Corrola though it has been giving me nightmares with regard to fuel consumption. Its average consumption has been about 9 kilometers per litre and a visit to various mechanics seem not to unravel the problem. I have undertaken service, changed spark plugs, changed the ATF oil, carried out service without any fruitful results. A sensor diagnosis revealed that everything is in perfect order. My last visit to the garage, the mechanic informed me that probably the fuel filter need to be changed. Another told me that the exhaust sensor is the problem and should be changed.
One strange aspect is that when the fuel goes below the reserve tank, filling up again the tank results in a lower mileage for instance normally 10 litres will give me approximately 90 kilometres but filling after the fuel reserve tank has gone below, it will take me a distance 50 or 60 kilometres.