2003 Pontiac Sunfire Lack of power, low milege

I have replaced the plugs, and cleaned up the top end module. 1 plug seemed fouled due to oil.I have a new fuel pump, and the injectors were cleaned and serviced 4,000 miles ago.
I have gone from 32MPG tp about 6MPG, the car is weak going up a hill, and has very little power. There is a slight whistle when driving, and the car sounds a little loud. First time here, and will donate if indeed my questions are answered. I do not like sending money info without t\knowing the receipent