Rear end ratio - Pontiac GTO Forum
User Tag List

Reply
 
LinkBack Thread Tools Display Modes
post #1 of 13 (permalink) Old 12-01-2013, 11:52 PM Thread Starter
 
Join Date: Dec 2013
Posts: 2
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Rear end ratio

I have a 1968 GTO that I am trying to determine the rear end gear ratio. I can not find any code on the axel. I am running a Munci 4-speed and the engine is turning 3,000 rpm at 60 mph. Based on the rpms and speed does anyone have a guess as to what the gear ratio is ? I was told it may be a 3:90 . Any help is appreciated.
classic68 is offline  
Sponsored Links
Advertisement
 
post #2 of 13 (permalink) Old 12-02-2013, 02:25 AM
 
Icefan71's Avatar
 
Join Date: Apr 2011
Location: Winfield, PA
Posts: 240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
I use this online calculator
MOTOR - A Rear Axle Calculator
I guessed your tire height (26.5"), it said 60.6440 mph with 3.90 gears.
You can also remove the rear axle cover and look at the numbers on the ring gear. You'll probably have to jack the rear up and rotate the tires until you see the nimbers. It will have a bigger#/smaller#. Divide and you'll get your ratio.
Icefan71 is offline  
post #3 of 13 (permalink) Old 12-02-2013, 07:16 AM
 
Join Date: Feb 2011
Location: Des Moines, IA
Posts: 662
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Garage
I think the easiest way to check is count the revolutions of the drive shaft on one complete turn of a rear wheel. Lift the rear end of the car off the ground. Transmission needs to be in neutral. Put a piece of masking tape on the drive shaft and a piece of tape on one of the rear wheels. Turn the rear wheel slowly one complete revolution and count the revolutions of the drive shaft. If you have nearly four revolutions, you have a 3.90:1. I suspect you will have about three and a half, in which case you have a 3.55:1. Regards, Matt

Matthew is offline  
 
post #4 of 13 (permalink) Old 12-02-2013, 07:52 AM
 
Join Date: Sep 2011
Location: west mich ... Grand Rapids Area
Posts: 95
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
^ what he said ( sorta )
1 lift the rear wheels
2 neutral
3 do both wheels spin when turned ?
4 if both turn same way you have posi and can mark the tire and axle and check ratio
5 if 1 tire spins forward and the other reverse you have an open carrier and need 1 tire
on the ground, mark the tire and axle. turn the tire 1/2 revolution and count axle
fasterfiero is offline  
post #5 of 13 (permalink) Old 12-02-2013, 10:03 AM
64-67 Expert
 
geeteeohguy's Avatar
 
Join Date: Feb 2008
Location: Fresno, California
Posts: 8,558
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 119 Post(s)
What Matt said. No way would you be running 3,000 rpm at 60 with 3.90 gears. More like 3300 rpm, at best. I suspect 3.55 gears, which were standard issue with the 4 speed cars. It was a different world then, with .29 per gallon 100 octane fuel. Today, for street use, 3.55-3.90 gears are not a good choice.
geeteeohguy is offline  
post #6 of 13 (permalink) Old 12-02-2013, 11:43 PM Thread Starter
 
Join Date: Dec 2013
Posts: 2
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Quote:
Originally Posted by Icefan71 View Post
I use this online calculator
MOTOR - A Rear Axle Calculator
I guessed your tire height (26.5"), it said 60.6440 mph with 3.90 gears.
You can also remove the rear axle cover and look at the numbers on the ring gear. You'll probably have to jack the rear up and rotate the tires until you see the nimbers. It will have a bigger#/smaller#. Divide and you'll get your ratio.
I am running stock 14 inch wheels. BF Goodrich 225/70/14 I measured the tire heigth and is was 24.5 in.
classic68 is offline  
post #7 of 13 (permalink) Old 12-03-2013, 07:11 AM
 
Join Date: Feb 2011
Location: Des Moines, IA
Posts: 662
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Garage
Looks like you are about to confirm you have a 3.55. If you are thinking about changing it, Geeteeohguy is providing good counsel here. Not the best gear ratio for the road today, but certainly better than a 3.90. Regards, Matt

Matthew is offline  
post #8 of 13 (permalink) Old 12-03-2013, 01:35 PM
64-67 Expert
 
geeteeohguy's Avatar
 
Join Date: Feb 2008
Location: Fresno, California
Posts: 8,558
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 119 Post(s)
With a 3.55 carrier, you can change to 3.36 gears if you can find them. They will make a big difference. You can also change to a 27" or 28" tall tire and that will help a bunch getting the revs down. I changed the 3.55's in my '65 to 3.36 's, and went to a 235/70/15 rear tire, and the car cruises nicely at 70 mph....a tick under 3,000 rpm and right in the power band. It did ok with the stock 14 inch tires, too, which for these cars are 26.5". Your 225/70/14 should be taller than 24.5"....that's a 60 series height.
geeteeohguy is offline  
post #9 of 13 (permalink) Old 12-10-2013, 03:45 PM
 
Join Date: Jun 2013
Location: Leeds
Posts: 44
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
I think you have 3.73:1

I had 24.5" tyres in my 67 GTO exact same tyre size as you, ran about 60mph at a fraction over 3000rpm. I have a M20 Muncie 4 speed with a 10 bolt 8" rear end. I fitted a 3.08:1 and it runs really well - far more drivable.


Sent from AutoGuide.com Free App
pcolenutt is offline  
post #10 of 13 (permalink) Old 12-10-2013, 06:43 PM
Former Moderator
 
ALKYGTO's Avatar
 
Join Date: Mar 2010
Location: Wentzville, Mo.
Posts: 2,893
Mentioned: 0 Post(s)
Tagged: 1 Thread(s)
Quoted: 16 Post(s)
Garage
Quote:
Originally Posted by geeteeohguy View Post
With a 3.55 carrier, you can change to 3.36 gears if you can find them. They will make a big difference. You can also change to a 27" or 28" tall tire and that will help a bunch getting the revs down. I changed the 3.55's in my '65 to 3.36 's, and went to a 235/70/15 rear tire, and the car cruises nicely at 70 mph....a tick under 3,000 rpm and right in the power band. It did ok with the stock 14 inch tires, too, which for these cars are 26.5". Your 225/70/14 should be taller than 24.5"....that's a 60 series height.
Agreed. I would try to find a taller tire and see what difference that makes.
ALKYGTO 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
Recommended rear end ratio. joesweeting 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions. 5 01-23-2013 06:38 PM
**Help** Rear End Gear Ratio 1967 Goat Oldsmobileman 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions. 6 06-10-2010 01:21 PM
Rear end ratio question jpdog357 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions. 6 06-21-2007 05:20 PM
rear end gear ratio question matt68gto 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions. 1 04-03-2006 11:30 AM
rear ratio's Jim D 2004-2006 GTO General Discussion 1 02-06-2005 02:18 PM

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