Thanks for the replys, gentlemen. I think I have discovered the problem. While looking on All-Data for some wiring diagrams to help me fix another problem, I happened to come across a TSB from 1992 that had the title, "Speedometer reading high when cold." So I checked it out. It said that when the outside temperature gets to be about 20° F, the speedometer will read high due to a lack of lubrication at the speedometer head. I thought about this for a minute, then it all clicked. When I bought the truck, it was still relatively warm out. Then, it got cold and the speedometer suddenly stopped reading right.
I then thought, "If this TSB had been around since 1992, why was the problem never fixed?" I then remembered some information from the dealer about the previous owner of the truck. The original (and only besides me) owner of the truck was our local Parks Department. They only drove this truck during the summer when it was warm out. So, they would have never noticed the problem. Even if they had noticed it, the truck was never driven on public highways so it would not have been worth the time or money to fix it.
Ford's reccomendation on how to correct the issue is to replace the speedometer head. Even if I could find a new speedometer head, the part costs $700. So, since I want to put a tachometer into the truck anyway, I'm going to find a factory cluster that has a tachometer already. This would kill two birds with one stone.
All of this leads me to a new question. Althought the truck was not originally equipped with a cluster that has a tachometer, is the wiring for a tachometer still present? Also, will the new cluster just plug into the old wiring harness and still function properly? Since the idea behind manufacturing vehicles it to build them as cheaply and efficiently as possible, I would think that the answers to my questions are "yes," but I'm not sure.
Any help is appreciated.
Joe, aka SuperKid