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New differential!! Need new speedo gears

Jake73

Member
Joined
Jul 26, 2006
Messages
7
Location
Newton
Corvette
1973 coupe
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
 

Jake73

Member
Joined
Jul 26, 2006
Messages
7
Location
Newton
Corvette
1973 coupe
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
 
A

aapple

Guest
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
 
V

vmrod

Guest
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

Member
Joined
Jul 26, 2006
Messages
7
Location
Newton
Corvette
1973 coupe
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

Well-known member
Joined
Jul 18, 2003
Messages
455
Location
Louisville
Corvette
1981
I have the 400 turbo and 3.55 gears, does it make a difference what tires? I have 255/60/15's.
 

CFI-EFI

Gone but not forgotten
Joined
Feb 12, 2009
Messages
19
Location
USA
Corvette
1984 Crossfire
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]
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
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
 

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