Tremec 3550 in a '91 GT, speedo questions

[not1337]

I recently purchased a '91 GT that has a 3550 in it. The dash speedometer bounces all around until it reaches around 50mph, then it's probably 5-10mph off.

I'm not sure where to begin here to diagnose. The car has speed control but it doesn't work, which might mean that the VSS is somehow involved? I have the dash out of the car right now anyway for an unrelated project (fire at starter solenoid/coil, need to replace harnesses) so getting to the cable from the cabin isn't difficult.

I guess what I'm asking is, is there something special that needs to be done to get a 3550 to either spin the cable at the appropriate rate to drive the speedo accurately, or is there some way to electronically drive the speedo by tapping a wire?

Thanks in advance.

 

[HotFox]

First off it sounds like your speedometer needs to be rebuilt as it has the typical ford speedometer bounce. The next thing is to figure the rear gear you have and buy the correct speedometer gear that is in the speedometer drive that inserts into the transmission. You can do a search as there are many calculators that calculate the correct gear needed based on you tire size and rear end gear. From what I found the 3550 transmissions came with 7 tooth drive gear. The 6 tooth gives you more selection depending on your rear gear but to change it you have to pull the tail housing off the transmission.

I just went though this on my stang with a 4.56:1 gear and a TKO 3550. I ended up buying a mechanical converter and made a speedometer gear change to correct the error in the speedometer. I need to have my 140mph rebuild yet as it has the bounce and it only has 50K on it I think. Even the 94’ F150 has a bounce at lower speeds. I think the cheapest I found to go thru a speedometer was like $250. Let me know if anyone knows a reputable place to rebuild a speedometer cheaper.

link to custom mechanical converter that I bought
Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4

scott