I had the same problem....when i bought the car i would average around 45mpg but recently I get on average somewhere in the 30's and if i throttle it i get mid 20's maybe less. I got told it can be a couple of things firstly the your temp sensor can be on the blink. This is cheap to replace the other problem that I know of is the more dearer one...one of your injectors mayb on the blink...you can try a injector cleaner when u next tank your car and see if it improves the mpg or one or all of them may need replacing.