dont the comp g's have 3.29 gears and get lower gas mileage? not sure if your a comp g.
it could need a tune up, and we are on the winter mix gas wise...have you been starting the car and letting it warm up for ~10mins? get the engine light checked for free at an autoparts store. you might need a new front o2.
dont just go by what the guy you bought it from said, he was selling you the car. you should investigate for yourself so you know whats from and make it worse.