I have a d100 tha had a 3 speed on the tree which I convereted 4 on the floor. I put 2 diff 3 speeds and now the 4 speed happy with the swap but getting the same problem. The speedo seems to be 5-10 miles off between 25-35 and once it hits 50-55 I have no clue how fast I am going.
Thanks

 

I would say that sometime in its life the rear gearing has been changed. And or a drastic tire size change from stockish sizes. Is it reading fast, slow? But the rear gear is needed to be known, then you need to start there and then figure the driven gear tooth count for your speedo cable...

 

The rear gear is matched with the speedo the truck is x air-force it only has 6,800 miles on it I have p235 -75-15 on it. Its reading slower than what I am actually going.
Thanks

 

Ive done the swap in other models. Pull the speedo gear in the tranny. Look in the book for other speedo gears for different rear ends. Use a gps unit with speedometer to figure out which gear gets you close. It wont be perfect but you can get it close.

 

Well ok, let me word it this way. The gears from trans to trans has apparently changed in the course of the swaps. the only way to fix the problem is to find out the gear ratio of the rear end and then re-gear the speedo cable and possibly change the drive gear IN the trans (if they can be changed in a mopar trans)... You never stated if it has been like that from the beginning (w/org trans) or after you changed the trans the first time. The reason i say this is if it was correct in beginning, something changed during trans swaps, and the drive/driven gears for the trans and speedometer, are the only thing it can be... UNLESS its a gummed up cable/housing which is an easy fix. The GPS idea is one way to be certain it is off.... M3C...