Can anyone tell me what to check,Speedometer works but has been jumping?
The main problem (Previous owner said 2 months ago)stopped calculating miles.
This is on 91 250.
I'm not sure if I need to tear dash a part or if this would be underneath truck.

 

If your odometer quit working you've probably lost the plastic gear in the speedo head. It's a common problem, here's a place that supplies the gears.

http://www.speedometergears.com/

As far as the needle jumping have you checked the sending unit at the transmission?

 

Thanks for the help,I may try my donor truck speedometer gears first....