Odometer and speedometer problems

Craw underneath the car. Towards the back of the transmission there is a connector and a couple wires going to it. Check the connection. If it is okay, try replacing the sender. It's the part with the connector.

If the speedometer is still working, but the odometer does not, most likely the cable is broken (separate drives for odometer and speedometer within a single cable) or the dash unit needs replacing...

could be the VSS - vehicle speed sensor .
Speedometer and Odometer
The Vehicle Speed Sensor (VSS) is a gear driven permanent magnet generator. The vehicle transaxle houses the VSS. The VSS generates a sine wave output with a frequency proportional to the vehicle speed.
The Powertrain Control Module (PCM) converts the CKT 389 signal to an output that switches to ground at a frequency of 4000 pulses per mile. The CKT 389 signal drives the speedometer and the odometer. Various other vehicle components use the CKT 389 signal. Refer to Engine Controls.

This is a fairly common problem on these old T-birds. According to legend, a factory in Germany made the little plastic gears that drive the odometer and they just made a bad batch of plastic. The gears last about 10 years and then turn to powder. If you get another speedometer from the salvage yard and it works, good luck, because it will probably die soon also. From a dealer....well, it's a **** shoot. In Orlando, Florida, there are several shops who have had new gears made and they can fix it for about $100 off the car. Try locally first, if it's so common then there are probably others who have new gears too.

fghf

my tach and speedo work without moving plus it adds mileage to my odometer without moving at all why
1988 ford thunderbird turbo coupe

You need to replace the VSS or vehicle speed sensor. Located on LH, lefthand rear of transmission.

Sheilah

speed sensor runs all three gages. Sounds like trans might have lose or missing bolts. Get it checked out.

The problem is going to be a worn or broken odometer gear. You can purchase the gear at odometergears.com for $29.00 with the shipping. Follow their how to repair on the Mustang to help in the repair. I believe you will have just the worm gear to replace.
http://www.odometergears.com/subpages/MustangHowTo.html

these can also be caused by a bad alternator