The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > 47 - Current classic GM Trucks > The 1947 - 1959 Chevrolet & GMC Pickups Message Board

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 04-03-2016, 09:40 AM   #1
jeffbcn
Registered User
 
jeffbcn's Avatar
 
Join Date: Nov 2015
Location: Las Vegas Nevada
Posts: 84
Differential highway geats

47 thriftmaster 1962 235 with Dragon Fire ignition. I believe it still has stock 4 speed floor shift and differential.
My question is what gear ratio in the differential would alow it to run at 65 or so on the highway? Right now it runs great and would probably go over 80 but I don't want to over Rev it. At 65 it runs 3,000 rpm. It gets about 10 mpg.
What RPM is recommended for cruising and what gears would I need to get it there?
Any comments would be appreciated.
Attached Images
 
jeffbcn is offline   Reply With Quote
Old 04-03-2016, 10:30 AM   #2
mongocanfly
Post Whore

 
mongocanfly's Avatar
 
Join Date: May 2015
Location: Alabama
Posts: 14,670
Re: Differential highway geats

what rear gears do you have now? If you have the trans 4th gear ratio, rear end ratio and tire size you can do the math and it will tell you what rear ratio you need for the rpm you want to run.
mongocanfly is offline   Reply With Quote
Old 04-03-2016, 12:15 PM   #3
jeffbcn
Registered User
 
jeffbcn's Avatar
 
Join Date: Nov 2015
Location: Las Vegas Nevada
Posts: 84
Re: Differential highway geats

I think 411. What is a decent highway rpm for a 235?
jeffbcn is offline   Reply With Quote
Old 04-03-2016, 01:37 PM   #4
mr48chev
Registered User
 
mr48chev's Avatar
 
Join Date: Jul 2007
Location: Toppenish, WA
Posts: 15,711
Re: Differential highway geats

Most guys go to the 3.55 gears. Several vendors I checked sell them usually around 450.00 for the ring and pinion. Here is a set on Ebay for 389 but you might want to do some more searching. http://www.ebay.com/itm/1940-1941-19...25.m3641.l6368

by checking one of the tire/gear/rpm calculators online you can figure out what your rpm drop should be at 65.

This one that Randy's ring and pinion has is easy to use and even gives the rpm for different trans ratios. You want the manual ratio which is 1 to 1
https://www.ringpinion.com/calculators/Calc_RPM.aspx
__________________
Founding member of the too many projects, too little time and money club.

My ongoing truck projects:
48 Chev 3100 that will run a 292 Six.
71 GMC 2500 that is getting a Cad 500 transplant.
77 C 30 dualie, 454, 4 speed with a 10 foot flatbed and hoist. It does the heavy work and hauls the projects around.
mr48chev is offline   Reply With Quote
Old 04-04-2016, 07:21 AM   #5
1project2many
Registered User
 
Join Date: Mar 2012
Location: Lakes Region NH
Posts: 3,200
Re: Differential highway geats

I found this article very interesting:

http://vcca.org/forum/ubbthreads.php...rear_gear_conv
1project2many is offline   Reply With Quote
Old 04-04-2016, 08:17 AM   #6
jeffbcn
Registered User
 
jeffbcn's Avatar
 
Join Date: Nov 2015
Location: Las Vegas Nevada
Posts: 84
Re: Differential highway geats

Interesting article but that was for a 37. Mines a 47, probably similar to the 51 mentioned.
jeffbcn is offline   Reply With Quote
Old 04-04-2016, 03:02 PM   #7
curmudgeon
Registered User
 
Join Date: Jun 2012
Location: San Francisco, CA
Posts: 83
Re: Differential highway geats

"Best" rpm for a stovebolt would be where your particular engine is putting out maximum torque, based on cam grind, displacement etc, (generally around 2500 rpm). Stock rear is 4.11, the 3.55 set is the only aftermarket replacement for the torque tube.

http://www.stovebolt.com/ubbthreads/...&Number=340589

The I-15's around Vegas are fast freeways, at "only" 65 mph I would be a moving roadblock. Many of us replace the torque tube with an open differential, I'm running a 3.08 rear which allows me to reasonably keep up with Calif freeway speeds. Of course you have a really nice original truck which you probably want to keep stock, so 3.55 gears are your best bet.
curmudgeon is offline   Reply With Quote
Old 04-04-2016, 03:22 PM   #8
OrrieG
Registered User
 
OrrieG's Avatar
 
Join Date: Jun 2006
Location: Idaho
Posts: 8,800
Re: Differential highway geats

Here is the dyno information for a 235. They are not a high rpm engine, HP peaks at 3800, torque at 2000.
Attached Images
 
__________________
1959 Chevy Short Fleetside w/ 74 4WD drive train (current project) OrrieG Build Thread
1964 Chevelle Malibu w/ 355-350TH (daily driver)
Helpful AD and TF Manual Site Old Car Manual Project
OrrieG is offline   Reply With Quote
Old 04-04-2016, 04:38 PM   #9
1project2many
Registered User
 
Join Date: Mar 2012
Location: Lakes Region NH
Posts: 3,200
Re: Differential highway geats

Quote:
Originally Posted by jeffbcn View Post
Interesting article but that was for a 37. Mines a 47, probably similar to the 51 mentioned.
True, but it provides some insight into another route that might be possible.
1project2many is offline   Reply With Quote
Old 04-04-2016, 11:19 PM   #10
jeffbcn
Registered User
 
jeffbcn's Avatar
 
Join Date: Nov 2015
Location: Las Vegas Nevada
Posts: 84
Re: Differential highway geats

Thank you all for the valuable information. It seems a 3.55 is best and drive it on the highway from about 1800 to 2000 rpm. Right now at 45 it's about 1900 rpm. Hopefully a 3.55 will take me up to 60 to 65.
jeffbcn is offline   Reply With Quote
Reply

Bookmarks


Posting Rules
You may not post new threads
You may not 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

Forum Jump


All times are GMT -4. The time now is 10:42 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com