1989 5.0 Gas mileage

You should get considerably better mileage than that. Mine gets 19 in town and 23 on the highway with the A/C on. Can't remember if the 89 has a check engine light or not but if the O2 sensors are bad then changing them would help the mileage some. Try cleaning you IAC, check your plugs, maybe new cap, rotor, wires. Check your timing.

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.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.

Or for a nicer scanner see http://www.midwayautosupply.com/detailedproductdescription.asp?3829 – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $35.

Im getting about what Bladez5_0 is getting for mileage with my 91.

Ive got an 89 engine/drivetrain in my 93 coupe, its a 5spd and I just got back from a trip yesterday evening and I got 24.5 mpg and I have 3.08 gears. The engine has 192k miles on it.

Do pull the codes, thats the best thing to do to see exactly what is going on with your car. It might be a little intimidating at first but its really not that hard to do.

i would check the plugs first. My 89 went from 15 to 20mpg hwy after I changes the plugs. Also clean the throttle body if you have'nt done so.

I get about 22 MPG with a 50/50 town and highway....and I have gotten 25+ on the highway

i get around 23-25 on the highway, not sure in town. 2.73 gears and a few mods. i would def. tune it up and change the o2's. see where that gets ya.