2004 Malibu very bad gas mileage

I recently purchased a 2004 Malibu with the V6 engine. It has over 104,000 miles. It was well maintained, oil change every 3,000 miles, etc. Engine seemed to run smoothly when I first drove the car. A handheld scanner detected a cylinder 6 misfire. I recently installed new spark plugs and wires. The compression in cylinder 6 is good, and the mechanic didn't mention a misfire. The gas mileage is still very bad. 15 MPG average for city and highway. Yesterday I added BG 44K fuel treatment incase the injectors are clogged. Any ideas?

i have an 07 malibu 4 cyl. and i expected better gas milage outta a 4 cyl....how do u drive ur car? u liek to speed? or take it nice and slow

I accelerate gradually, and don't drive fast. I am being especially careful because I am trying to get better mileage.