Speedometer replacement

[89OctaneStang]

Well, I just replaced my 85 MPH speedo with a 140 MPH speedo. But now I'm having problems getting it to read right. I changed my original 18 tooth gear set to a 23 tooth gear set and it didn't fix the problem. And when I took the 23 tooth gear out today, the gear teeth had been broken while inside the trany during that short period of time. Has anyone worked on their speedo before and experienced something similar to this and found the fix to the problem? Cause I'm clueless at this point....

 

[TRWXXA]

Well, I just replaced my 85 MPH speedo with a 140 MPH speedo. But now I'm having problems getting it to read right. I changed my original 18 tooth gear set to a 23 tooth gear set and it didn't fix the problem. And when I took the 23 tooth gear out today, the gear teeth had been broken while inside the trany during that short period of time. Has anyone worked on their speedo before and experienced something similar to this and found the fix to the problem? Cause I'm clueless at this point....

You DO NOT replace the speedometer gear when changing the speedometer. The speedometer gear is only changed when changing the rear end gear ratio or tire size. Put the original gear back in.

 

[Killer50stang]

You DO NOT replace the speedometer gear when changing the speedometer. The speedometer gear is only changed when changing the rear end gear ratio or tire size. Put the original gear back in.

Just what I was thinking....all it is is a backplate...it should just work...but keep in mind it reads different.

 

[89OctaneStang]

I didn't change the speedo gear right away. I only changed it after I went down the road for a test drive to see how the new speedo worked and it was telling me I was doing 130MPH when I was actually doing about 45 MPH. If it isnt the gear set then what would cause that? The 85 MPH guage worked perfectly before I put in the 140 MPH guage.

 

[TRWXXA]

I didn't change the speedo gear right away. I only changed it after I went down the road for a test drive to see how the new speedo worked and it was telling me I was doing 130MPH when I was actually doing about 45 MPH. If it isnt the gear set then what would cause that? The 85 MPH guage worked perfectly before I put in the 140 MPH guage.

That shouldn't happen. The 85 mph, 140 mph, and 160 mph speedometers all work on the same standard input -- 1000 rpm/mile. Swapping one speedometer for another is all you have to do.

Is your 140 mph speedo a new unit, or a used one? It's possible that it's broken, or had the 85 mph instrument face changed to a 140 mph face. There is no speedo gear that can correct that much error by itself.

 

[89OctaneStang]

No, it was a used unit. So I guess that unit is broken then. Thanks for the input....

 

[TRWXXA]

No, it was a used unit. So I guess that unit is broken then. Thanks for the input....

Ouch! I hope you didn't pay too much for it.

You could take it to a speedometer show and see if they can fix it. But it would probably cost as much as getting a new unit. I suggest just biting the bullet and getting a new speedo -- $160 to $180, from Latemodel Restoration Supply. It just seems that with used mechanical parts you always wind up buying two, just to get one that works.

 

[89OctaneStang]

I see that...

 

[cenok is family]

did you replace the whole instrument cluster or just the speedo itself?

 

[89OctaneStang]

Just the speedo

 

[89OctaneStang]

The used speedo I got didnt have the little black cap that goes on the needle. When we attempted to use the black cap off of a tachometer that wasnt being used it would read funny because the cap had a little weight in the back of it. Could the speedometer be messed up cause of the little black cap being gone?? If anyone has one laying around, do me a favor and pull off the black cap on the needle and look to see if there is a little chunk of mettle in it, please....

 

[TRWXXA]

The used speedo I got didnt have the little black cap that goes on the needle. When we attempted to use the black cap off of a tachometer that wasnt being used it would read funny because the cap had a little weight in the back of it. Could the speedometer be messed up cause of the little black cap being gone?? If anyone has one laying around, do me a favor and pull off the black cap on the needle and look to see if there is a little chunk of mettle in it, please....

It's possible. The speedometer is a pretty sensitive instrument -- only driven by a small magnetic field and a light spring. Changing the balance of the needle could throw it off quite a bit.

Sorry I don't have a speedometer around to check on that cap.

 

[cenok is family]

i got an old 85mph speedo i can check the cap on that, dunno if it's the same though. i just did the same thing to my car a couple weeks back. had to do a **** load of trimming to get it to lay in the cluster flush, but all the mechanicals work perfect without changing anything.

 

[89OctaneStang]

Well I fortunately didnt have to do any trimming to get it to fit but I am having trouble getting it to read right. I just went to the guys house that I bought the 140 from and picked up my old 85. I checked the back of the needle and sure enough, it has a small little piece of metal or magnet in it. So, after work tomorrow, I'm going to tear apart my dash again and get the 140 out and put on the needle from the 85. I hope that the MPH isnt gaged by the specific weight of the needle. But I will have an answer for everyone very soon.

The next thing I'm working on is my rear differential traction lock clutch that is burned out. Anyone have any experience with this???