2002 Chevy Impala fuel mileage is terrible

You should scan your car for codes, and take a look at your STFT and LTFT.
Also, when was the last time you: Put a new air filter? Change your spark plugs and wires (it did the trick for my wife's impala)?
Make sure your exhaust is not plugged

Other suggestions:
PCV valve
Coolant sensor
Fuel pressure regulator

I have changed plugs, wires, fuel filter, air filter, flushed coolant, checked exaust. Plugs only had about 8 thousand miles and were burnt pretty bad. Put in new plugs now the car runs rough. Yes I checked the order and made shure the wires were all the way on. Still looking for answers

If your plugs were burnt at 8,000 miles, look no further. This is your problem right there.
Do you have any DTC?
Now you have to find out what causes your plugs to go bad so quickly:
What did the plugs look like when you replaced them? Were the electrodes melted? (check here for help on diagnosing:

http://www.2carpros.com/articles/mitchell1eautorepair-car-repair-manuals