Erm.. did u change your standard rims into 17 inches, heavy ones?
Common things to check is :
Engine oil - must change every 5000 km. Alot of ppl like to drag until 30,000 km. It is NOT recommended. Even if you change to fully synthetic oil also need change, max 10,000 km oni.
Air filter - May be clogged up therefore por air flow = poor combustion = power loss
Spark plugs - Oil may have cover the plugs and lots of carbon stuck on it. Check or replace if necessary. Not expensive to change around RM40 or less for an NGK.
Auto Trans. Oil - Must replace every 20,000km. All auto cars must regularly change gearbox oil coz very sensitive. Unlike manual trans., auto geabox must take extra care or else extra money need to get ready - to change gearbox. :P
Roof rack - DO you have it installed on your car roof? It gives more resistance while driving therefore slows down your car = hgher fuel consumption.
Heavy load in boot - Make sure you dont have heavy barang in your rear boot, it slows down your car too.
Mileage - How much is your engine mileage. You may have bought the car 2nd hand where the previous owner really squeezed the engine to the max, although you MAY have low mileage. You never know ... Therefore, when u got the car, its already exhausted and its time for a new engine.
Thats my few cents of advise. You may not follow directly but you can take note on the above points.
GOOD LUCK! ;)