Overdrive? - Pontiac GTO Forum
User Tag List

 
LinkBack Thread Tools Display Modes
post #1 of 2 (permalink) Old 01-19-2018, 07:33 PM Thread Starter
 
Join Date: Jul 2016
Location: NW Ohio
Posts: 29
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Overdrive?

Good Evening All

It's been a while since I've posted; been working on a couple of vintage lawn tractors. Now that we are in the depths of winter, I've turned my attention back to the cars.

I would like to slow down the rpm's of the wife's '66 GTO tri-power 4 speed. The car is stock down to the 14" wheels and the 3:55 posi. Modifying the sheet metal is not an option.

I'm looking at either replacing the 3:55 with a 3:08 or a 4 speed overdrive if one is available that will bolt up to the stock bell housing.

What about a stock 3 speed overdrive from the late '50"s?

As always, all thoughts and comments are welcomed.

Omni
Omni is offline  
Sponsored Links
Advertisement
 
post #2 of 2 (permalink) Old 01-20-2018, 10:02 AM
 
PontiacJim's Avatar
 
Join Date: Dec 2012
Location: Gastonia, NC - Born & raised in Connecticut - 31 years
Posts: 3,648
Mentioned: 1 Post(s)
Tagged: 1 Thread(s)
Quoted: 390 Post(s)
Garage
Quote:
Originally Posted by Omni View Post
Good Evening All

It's been a while since I've posted; been working on a couple of vintage lawn tractors. Now that we are in the depths of winter, I've turned my attention back to the cars.

I would like to slow down the rpm's of the wife's '66 GTO tri-power 4 speed. The car is stock down to the 14" wheels and the 3:55 posi. Modifying the sheet metal is not an option.

I'm looking at either replacing the 3:55 with a 3:08 or a 4 speed overdrive if one is available that will bolt up to the stock bell housing.

What about a stock 3 speed overdrive from the late '50"s?

As always, all thoughts and comments are welcomed.

Omni
3 speed OD from the late '50's would probably not hold the torque and no doubt fitment issued would be a real problem.

A 4-speed with OD is called a 5-speed. Several out there like TKO or Richmond. The TKO requires the floor to be cut and modded due to it's size and I don't think the Richmond does - but check into that. The TKO uses 5th gear as an overdrive and you have to go up with the rear gears to around a 3.70 - 3.90 range to really take advantage of it. The Richmond on the other hand is a 1:1 5th gear, but has lower gearing 1-4 so you would need to drop the rear gear ratio down to something like 2.70's to really get the benefit. Here is a neat comparison: http://www.5speedtransmissions.com/5sp_comparison.html

Not an inexpensive swap.

I would not swap to the 3.08's. Reason being is that it will take more clutch slip to get the car rolling and is a real pain taking off on inclines or hills. It would be an improvement over the 3.55's, and can work, but you'll be using up a lot more clutch disc. You will also lose some performance, of course.

You did not say if you still have the factory 10.75 compression. The 3.08's will lug the engine down a little more and you may experience detonation/pinging so timing may have to be adjusted. Assume if 10.75, you are already using a good grade of high octane gas.

Last edited by PontiacJim; 01-20-2018 at 01:32 PM.
PontiacJim is offline  
Sponsored Links
Advertisement
 
Reply

  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions.

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Pontiac GTO Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Similar Threads
Thread Thread Starter Forum Replies Last Post
M-21 Overdrive Gearset Brad3112 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions. 1 05-03-2017 05:23 PM
Cheap overdrive swap? Orion88 1964-1974 Tempest, Lemans & GTO General Discussion 4 01-14-2014 08:59 PM
Cheapest way to get overdrive xconcepts 1964-1974 Tempest, LeMans & GTO Technical and Electrical Wiring 68 08-31-2010 03:19 PM
overdrive danthepontiacman 1964-1974 Tempest, LeMans & GTO Technical and Electrical Wiring 0 04-22-2010 01:50 PM
Automakers in overdrive 05GTO In the News 0 01-07-2005 04:56 AM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome