speedomoter problem, need help
 

So I left the transmission end of my speedo cable unhooked (while driving )like a knucklehead and it boogered up the end.

Got a new cable and hooked up both ends, should be no problem right? wrong.

I get a mile down the road and my speedometer starts making a whining noise and the needle rises up to like 80mph (when I'm doing 20-30) and then goes back down and starts working normally again. Turned the truck around and it does it again before I get back home. I haven't investigated yet, but I'm pretty sure I screwed both ends on real good.

What the heck is going on? anyone experience this before?

I used to have a 72 blazer that had that little gearbox thing where it hooks into the transmission but this truck has never had that (71 Jimmy).
Thanks.