1995 Mustang: I HAVE A 1995 FORD MUSTANG CONVERTIBLE. ALL OF A ...

Sounds like you know your way around, here is a link to some speedo problems:

2000 Windstar Speedometer Quit Ford Forums Mustang Forum Ford Trucks Ford...

Looking for widespread problems I did find this:
http://circuitboardmedics.com/search.php?search_query=windstar&x=0&y=0
I used these guys for a Rav4 ECU problem, really good.

Here is a vid:

Speedometer gear in the back of the transmission tail piece. remove the cable going to it, then twist and pull out the gear. Good luck finding the gear for the instrument panel if it is broken.

Based on what you wrote here, it sounds like your ECU is going out.

This is a pretty common issue with the 2005 Mustang speedometers. The reason it's acting up is because the motor that controls the gauge is going bad. There's a DIY guide on how to repair it here - 2005 Mustang Speedometer Problem

Also, you can send it in for repair to a speedometer repair specialist.

- Dr.Speedometer

This is pretty common on the 2005 Mustangs, it's caused by failing stepper motors in the instrument cluster, they control the movement of the gauges. Repair is around $180 and there's a guide on how to fix them yourself here - 2005 Mustang speedometer problem

check the fuses

The gear for the odometer has broken. If you are mechanically inclined you should be able to replace the gear yourself. Google mustang odometer inop .You can buy the gear. Good luck.

Could be a fuse my old sunbird would tell me it was going 60km an hour at a dead stop and when i started moveing it would drop to 0 replaced ALL the fuses and had no more problems

Modern cars no longer use a mechanical cable and speedometer. They have an electronic sensor called the vehicle speed sensor (VSS) which is mounted to the transmission where the old speedometer cable used to go. If the engine was replaced, perhaps the mechanic forgot to plug the wiring harness back into the VSS? Or perhaps he damaged the wiring or the sensor during the engine overhaul. (The transmission has to be removed to remove the engine from the car.)
Charlie

the instrument cluster needs repair, the steppers on the motor need upgrade nothing else or buy a new instrument cluster