Speedo Cable and speedo gear box

[guyryan100]

Mr48 is on to something - I recall now having the exact problem and it was caused by the wrong housing. I had a 39 tooth gear (brown) and I had a housing for 40-45 gears.

Its easy to test this gear when you have the speedo cable off and the driver side rear tire lifted off the ground, truck in gear, you can turn the tire with your foot and watch the gear turn. You can push your finger against the end of the gear to create resistance on the gear to confirm its turning under resistance.

In your case the gear definitely isn't turning, and the housing could be the reason.