I just rebuilt my TH400 in my 82 Chevy C10. I put a brand new speedo cable in with it. The needle now jumps in my gauge and it seems like im not going the correct speed limit even though it says i am (im going slower). Plus after 3rd gear it sounds as if im going into high RPM's. Any help? Its driving me nuts

 

I believe those cables have different gear ratios on them for different applications. I'd suggest a tachometer while driving to verify what rpms in all gears, it should of course drop the rpms as you reach higher gear ratios.