I have a 2000 f250 Superduty that has sat for a while. The speedometer now goes fine from 0 to about 15 and this jumps up to 75 and stays there. Can anyone anyone tell me what to check?

James

 

Please check my signature below so I can submit correct diagrams/instructions.

 

this is the 2000 super duty f250 with the v10 triton and 4wd.

 

Since you never mentioned if your ABS (yellow) light stays on, I assume it proves out correctly during bulb check at start up. Also, do you know you have 4WABS (4-wheel abs) or RABS (2 rear wheel abs) or you are not sure?
Let's start from the easiest tasks first by cleaning the electrical connectors of your OSS and TSS. Clean both ends using electrical contact cleaner, if they are in poor condition, then use 91% isopropyl alcohol. Download the last 2 zip files to get their location and diagram.
If that doesn't help go over Pinpoint Test H, all those zip files are connector location and diagram needed on running pinpoint H.

C240B - GEM
C242B - Central Junction Box (CJB)
C250B - Instrument Cluster
C1040A - 4WABS

And finally also go over Speed_Sensor-Service_Repair.pdf.