10 mpg on highway? WTF??

you replaced the ECT (vs the gauge sender), i take it.
how about testing the ACT? lambdas?
those are the biggies that come to mind. FWIW, in town i get 15 mpg or so. it really sucks.
good luck.

What kind of maf is it?

Replace O2 sensors and you will get your fuel mileage back.

I have the same problem I get about 165 miles to a tank. Highway driving is no better and it doesnt matter if I drive fast or slow!

Thats how mine is. Even after the gears it dident get worse. But its been horrible.
Now that my speedo dosent work I dont even MOVE!!!!!

Pull the codes before you start replacing sensors - they'll tell you if you're having a sensor issue.

Dump the codes and see what the computer says is wrong…

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.

See http://www.troublecodes.net/Ford/
OR
See http://www.dalidesign.com/hbook/eectest.html for more descriptive help
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Walmart.