lousy mileage on 2004 Ford Taurus

My miles per gallon has dropped from 19 or 20 mpg to 16 or 17 miless per gallon, why?

1 answer
I would check your basics, i.e. how are your tune up parts i.e. spark plugs, filters? Do you have a Service Engine Soon Light on ? This could mean that an engine sensor is not working properly which will really effect your mileage.