Horrible Mileage on a 99 Taurus. Why?

I have a 99 Taurus SE (the 200hp one), 103k, and it gets about 10-12 MPG (about 180 miles on a tank). This has become extremely frustrating as I've done everything I know to get the mileage up, I put in a new Air filter, new plugs (tight), pcv's good, I don't run the A/C (even though it is 90 in December here in South Florida), new full tires, etc. I put in a new speed sensor a little while ago that solved some major shifting problems. But for some reason, the RPM's are always extremely high (4,000 @ 60), and when I drive in overdrive, as I always do (still with crappy mileage), that brings the rpms down about 1,000. The only code I've gotten out of it is for some swapped downstream sensor.
I know this car was rated for 19/21 mpg in 1999, and that was 8 years ago, but still, 10/12 is unacceptable.
I'm desperate for any help you can offer, even though I want some type of instant cure for my car, thanks!