New differential!! Need new speedo gears

[Jake73]

Thought I would see if anyone has had to do this. I had new rear diff. gears installed. Went from 3.08 to 3.55 with auto trans. I need to find the right driven gear for my speedometer to read correctly, I think. Ecklers and the sort carry them but how do I know which one is right there are about a dozen or so. Calculators say something about the tranny gear size to find the right driven gear along with tire size and rpm but I can not seem to find what a th400 has for gears that run the speedo. The speedometer seems to be running fast, I haven't had a chance to find out exactlly, showing I am going faster than I reall am. Any help would be greatly appreciated. Thanks

 

[black_81_vette]

search the date 2-28-06 I have a post covering what you need to fix it

 

[Jake73]

Thanks for the reply black 81 vette. I am still trying to find out what the stock drive gear for the TH400 is with the 3.08. That is the only info I am missing. Do I just need to look at what I actually have or is there a way to find out without taking thing apart to look? Thanks

 

[aapple]

Stock 400 trans w/3.55 rear takes a brown, 15 tooth drive gear, and a black, 40 tooth driven gear.

Stock w/3.08 is 15/35

 

[80VetteL82]

Stock 400 trans w/3.55 rear takes a brown, 15 tooth drive gear, and a black, 40 tooth driven gear.

Stock w/3.08 is 15/35

How about a TH350 with 3.70s?

 

[Jake73]

Thanks aapple for the info.

 

[vmrod]

Is there an O-ring or something that is supposed to go in there somewhere?

I put in a TCI transmission and had used my speedo gears in the tranny. The cable is all hooked up, and it leaks ever so slightly.

I didn't mean to hijack the thread, but figured since ya'll are working in the area...

 

[Jake73]

vmrod, you might check somewhere like ecklers or the like. I am sure there is a seal but only for the 4 speeds for 77's. 68-75 have seals for the autos but I am not sure for the 76 on up. It looks like 77-82 years have a different cable or just one cable, the earlier years I think actually have 2 cables, an upper and lower. Not sure if this helps or not.

 

[LanceB]

I have the 400 turbo and 3.55 gears, does it make a difference what tires? I have 255/60/15's.

 

[bozzman3]

How about a TH350 with 3.70s?

Try a 22 or 23 tooth

 

[CFI-EFI]

thought i would see if anyone has had to do this. I had new rear diff. Gears installed. Went from 3.08 to 3.55 with auto trans. I need to find the right driven gear for my speedometer to read correctly, i think. Ecklers and the sort carry them but how do i know which one is right there are about a dozen or so. Calculators say something about the tranny gear size to find the right driven gear along with tire size and rpm but i can not seem to find what a th400 has for gears that run the speedo. The speedometer seems to be running fast, i haven't had a chance to find out exactlly, showing i am going faster than i reall am. Any help would be greatly appreciated. Thanks

you will need to replace the red, 17 tooth drive gear with a grey 15 tooth drive gear part number [size=-1]8642620.you can keep the green,42 tooth driven gear.

Race on!!!
[/size]

 

[JohnZ]

you will need to replace the red, 17 tooth drive gear with a grey 15 tooth drive gear part number [SIZE=-1]8642620.you can keep the green,42 tooth driven gear.[/SIZE]

[SIZE=-1]Race on!!![/SIZE]

This thread went dormant three years ago. :eyerole

 

[CFI-EFI]

this thread went dormant three years ago. :eyerole

the OP need may no lonfer NEEDthe info, but the info is correct and current.

Race on!!!