1985 Pontiac Parisienne

Engine Performance problem
1985 Pontiac Parisienne V8 Two Wheel Drive Automatic 115000 miles

Currently my 1985 Pontiac Parisienne Station wagon fuel consumption is 10/11 mpg city and 14/15 mpg freeway. I've been doing research and I read on line that this car should have at least 16 mpg city and 21 mpg freeway. How can I fix this car to it's original fuel consumption?
1985 Pontiac
November 1, 2009.

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