1966 GTO gear change - Pontiac GTO Forum
User Tag List

 
LinkBack Thread Tools Display Modes
post #1 of 2 (permalink) Old 09-19-2017, 05:23 PM Thread Starter
 
Join Date: Jul 2016
Location: NW Ohio
Posts: 29
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
1966 GTO gear change

Good Day All

Have a 1966 GTO 389 tri-power 4speed (Muncie). The car currently has a 355 gear, with 255x14 tires. The rear is a 10 bolt

The car is used mainly for cruising, no completion driving

I would like to drop down to something like a 308 to slow down the rpm's at cruising speed.

Any thoughts on this idea.

Also any thoughts on what speedo gears this would need?

Thanks to all who respond.

Omni
Omni is offline  
Sponsored Links
Advertisement
 
post #2 of 2 (permalink) Old 09-19-2017, 06:15 PM
 
PontiacJim's Avatar
 
Join Date: Dec 2012
Location: Gastonia, NC - Born & raised in Connecticut - 31 years
Posts: 3,652
Mentioned: 1 Post(s)
Tagged: 1 Thread(s)
Quoted: 390 Post(s)
Garage
Quote:
Originally Posted by Omni View Post
Good Day All

Have a 1966 GTO 389 tri-power 4speed (Muncie). The car currently has a 355 gear, with 255x14 tires. The rear is a 10 bolt

The car is used mainly for cruising, no completion driving

I would like to drop down to something like a 308 to slow down the rpm's at cruising speed.

Any thoughts on this idea.

Also any thoughts on what speedo gears this would need?

Thanks to all who respond.

Omni
Assume the 255 x 14 tire is about 26" tall. It could be shorter if you have wider tires, but lets go with 26" as that was factory size.

3.55's at 70 mph = 3,211 RPM's. Dropping to a 3.08 at 70 mph = 2,786 RPM's or a drop of about 425 RPM's. Not a whole lot, but better. Of course this is for highway cruising. Secondary and back road cruising RPM's would drop accordingly. A good speed/RPM match would be driving your car @ 55 MPH with the 3.08's which will put you at 2,189 RPM's which is a good number in my book. If some wise guy decided to give you a run, a quick throw into 3rd gear at 55 mph would put the engine nicely at 3,191 RPM's giving you an explosive pull going into 4th gear and shutting down your competitor.

However, there may be a down side. 3.08's are a bit stiff for a 4-speed on the take-off. You may have to use more clutch slipping than you are used to with the 3.55's. I had 3.23's and a HD 3-speed trans (a little lower first gear than the Muncie 4-speed) and went with a taller tire which effectively drops the rear axle ratio and my car acted more like 3.08's. I did have to use a little more clutch slip, especially on an uphill take-off, but it was not a problem. You also want to make sure you have a good pressure plate and disc that will not slip because the gearing is too stiff and the engine's torque overpowers the set-up. Learned that lesson when I was a kid and kept installing NAPA rebuilt clutch kits in my '67 GTO. Got so fed up replacing clutches (and money was also very tight) every couple months that I pulled the close ratio 4-speed and installed an automatic. So keep this in mind.
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
'65 GTO Rear gear ring and pinion change. tigervette 1964-1974 Tempest, Lemans & GTO Exhaust, Suspension, Alignment, Fuel Tank and Brakes. 1 04-06-2015 03:12 PM
Distributor gear change WhittP 1964-1974 Tempest, LeMans & GTO Technical and Electrical Wiring 2 08-01-2014 09:16 AM
'67 GTO rear end gear ratio change 67gtoconv 1964-1974 Tempest, Lemans & GTO Exhaust, Suspension, Alignment, Fuel Tank and Brakes. 2 04-19-2014 11:47 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