Speedometer quits after 50

I'm having a weird problem with the speedometer. Once it hits 50mph, the needle slowly starts to drop down to zero. Over the past few weeks, I've noticed that it's pretty consistent at the speed in which this happens. I just installed a new cable and while that steadied up the needle movement, it still falls back to zero. What I'm trying to figure out is if the problem lies in the speedometer or in the transfer case. Is there anything I could try first before I start replacing parts? Thanks in advance.

 

Sounds like a speedo gear that is worn to me. They aren't too much trouble to change.

 

You could try driving the speedo cable with your variable electric drill, and that would indicate if the gear or the speedo head is the problem. Disclaimer - I don't know if that's fast enough to get to 50, but I'd guess it is.

 

For standard speedo calibrations, 1000rpm on the cable = 60mph. An electric drill should be able to do it.

 

Thanks for the suggestions. If it is the speedo gear, are those still available for the D20?