I have been putting this off for years but now I am determined to fix the speedo on my 94 sidekick.  When it broke, I was driving along and all of the sudden the needle pegged to the right and a LOUD grinding sound ensued.  The needle popped off the peg and is lying inside the gage cluster case.  When it gets cold out, the grinding sound comes back. 
I have read up on this subject some, but i am still not sure if I need just a new speedo cable or the whole gauge assembly. 

If i get the whole gauge assembly does that mean odometer and all?  i really want to keep my current mileage reading. 

Please clarify what parts I actually need to replace.

Thanks

Jon

Well lets start at the beginning. Let's see what you need first.

Remove the cluster to get to the speedo cable. Remove from the side of the t-case and unroute it from the vehicle. Laying on the ground you should be able to make a big U of it and turn the cable inside. It should move in and out and rotate freely with no binding or sticking. If it doesn't you have to replace it.

Now remove your speedo gauge form the cluster. Place the needle at "0" push it on. Gently rotate the needle to 30 mph let go, it should go back to "0". If it does go to 60 mph. Repeat through the MPH range. If it does not come back to "0" you will be changing it out.

MY

That's almost certainly a gauge problem.

Inside the gauge the cable rotates a magnet, that is a very close to a disk that is attached to the needle - if the two come into contact with one another, you get that grinding noise and the erratic needle movement.

The one I got is from a 94 tracker.  The plug ins and the electrical circuits do not look the same as mine.  Can exctract just the speedo or do I have to use the whole cluster.  If I have to use the whole cluster it won't work.

You can pull just the speedo out and swap it over.  It's going from the 96 and later to a 95 and earlier that you run into issues.