Osama

I have Dihatsu Hijet 660cc S321v (Purchased recently) According to Meter it is 38000 km done. but they give me only 11-12 km per litre. w/o AC and with AC it gives me only 9-10 km per litre( acceleration / Pickup highly goes down when AC is on) Noted in city. please give me advise that how could i increase its mileage. i am recently change its sparks plug ,Air filter and Oil too. Also please told me that how could i check Transmission oil in it.means kaise pata chalega k transmission oil poora h ya nahe.