Poor Gas Mileage on 2000 Chevrolet Malibu

I have the V-6 engine and the gas mileage I think is not good ! Why is this and what could I do to fix it?The manifold gasket is leaking now ,what that cause poor mileage?

Asked by for the 2000 Chevrolet Malibu
I have the 3.1 and get around 33 miles on open highway and about 27 in city. a bad camshaft sensor will cause it to use more fuel then usual. the camshaft sensor on the 3.1 is under power steering pump.to remove. take off power steering pump and set aside, do not remove the hoses. remove the sensor bolt and pull out after removing the wire from it.make sure airfilter is in good condition.
2 answers
A leaking intake manifold (vacuum leak) can cause this, as well as weak oxygen sensors, plugged air or fuel filters to name a few. Have the codes checked too!.