2002 Oldsmobile Alero

Engine Performance problem
2002 Oldsmobile Alero 6 cyl Front Wheel Drive Automatic 85000 miles

I have 2002 olds alero with 85,000kms on it I bought the car used it had 47,000 kms on it I have switched it to synthetic oil replaced wires, plugs, fuel filter, air filter and im getting about 300 to 400 km per tank no check engine light coming on. Any ideas on what else I can do to improve fuel milage. Its a 3.4 litre. All parts are from gm.
March 8, 2008.

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