swapping to a th400 question

[h300r]

just wondering if i need to purchase anything to get my speedo to work properly i still have 3.42 in the rear any help is appreciated

 

[SSedan64]

You'll need the correct Speedo gears for the TH400.
http://www.transmissioncenter.net/speedometer_calibration_______va.htm#400
http://www.bgsoflex.com/speedo1.html

 

[h300r]

do u know what speedo gear i will need i have a stock rear

 

[SSedan64]

No way to know untill you check & see which Drive & Driven gears are in the Trans.
You can remove the Speedo Bullet/Housing, count teeth on Driven gear, look thru the hole & count teeth on Drive gear. This will give you a starting point. You may only need to change the Driven gear.
This site has good Info and GM Part #s, >> http://www.chevellestuff.com/tech/speedo.htm
local dealer may have what you need or check site in Post above or >> http://www.partszoneonline.com/index.php
OEM parts locater >> http://www.partsvoice.com/

This Pic is a TH350/700R4 Speedo gear diagram, TH400 is similar.