Gear caluclation? How to keep ratio's same as factory with big tires? - Jeep Wrangler Forum
Jeep Wrangler Forum

Go Back   Jeep Wrangler Forum > TJ Jeep Wrangler Forum > TJ Tech Forum

Join Wrangler Forum Today


Reply
 
Thread Tools

Please support our sponsors and let them know you heard about them on WranglerForum.com
Old 03-18-2008, 03:17 PM   #1
Jeeper
 
Join Date: Jan 2008
Location: Birmingham, AL
Posts: 174
Gear caluclation? How to keep ratio's same as factory with big tires?

I know this subject has been worn out time after time but I have to ask anyway. Is there a way to caluclate exactly what gear ratio I would need for different size tires in order to be close to factory conditions. I have seen a few charts but I did not know how to read them. I have read many peoples opions on what gears would be best for certain size tires based on their engine and transmission. However, I would like to know how to calculate what gears I need to buy if I stay with my current tire size or if I decided to step up a size.

To be a little more specific, I posted a few days ago and everyone agreed that I could fit 35's on my Jeep. I want to go this route but know I will have strengthen the axles with a kit and change gears. I am thinking about the super 35 kit and don't know what gears to go with. I am not trying to have 3 crawl gears and 2 overdrive gears, I would like to have 5th gear back on the highway and be able to crawl?

Forgot to add: 4.0 and 5 speed manual, currently 33's with 3.07

Thanks for the input.

__________________
1998 TJ Sport, 4.0 I6, 4.88's, super 35, Auburn lockers f/r, 7" RK, 36 Iroks, 15X10 ProComp series 51, K&N, swaybar disconnects, Ramsey 8500lb winch
branty is offline   Quote Quick Reply
Old 03-18-2008, 03:37 PM   #2
Jeeper
 
compshooter's Avatar
 
Join Date: Jun 2006
Location: 29 Palms, Ca
Posts: 407

__________________
99 SE with less than 43K on the clock and a few mods......
compshooter is offline   Quote Quick Reply
Old 03-18-2008, 04:13 PM   #3
Knows a couple things...

WF Supporting Member
 
Jerry Bransford's Avatar
 
Join Date: Nov 2005
Location: Escondido, Calif.
Posts: 30,852
Images: 2
One thing to keep in mind, which is a very important thing. Gear charts that show RPMs only tell you part of the story and if you use nothing but a chart, you'll be disappointed with the outcome. You actually need to regear to obtain slightly higher RPMs at the same speeds with bigger tires than you had previously with smaller tires.

This is because the bigger tire is harder to turn for these reasons... it weighs more, it has increased rolling resistance, the larger diameter has a longer radius that gives it greater leverage against the axleshaft trying to turn it, and that a lifted Jeep with big tires sits higher up in the wind stream so it has to fight harder to push itself down the highway at 65-70 mph.

So don't be afraid to go with a ratio that gives slightly higher RPMs than you had before, you'll need them. No one ever complained they regeared too low but plenty, me included after regearing to 4.10 for 33" tires, have complained they didn't gear low enough. That's why I went to 4.88 for 35" tires, I was sick of being undergeared.
__________________
When you have a choice, buy American.

Jerry Bransford is offline   Quote Quick Reply
Old 03-18-2008, 05:03 PM   #4
Jeeper
 
bpowa's Avatar
 
Join Date: Jul 2007
Posts: 836
Quote:
Originally Posted by compshooter View Post
i think that chart is wrong.. I have 3.73 gear with 32' and I am cruising at 1900-2000 rpm at 65mph.
bpowa is offline   Quote Quick Reply
Old 03-18-2008, 05:17 PM   #5
Knows a couple things...

WF Supporting Member
 
Jerry Bransford's Avatar
 
Join Date: Nov 2005
Location: Escondido, Calif.
Posts: 30,852
Images: 2
Quote:
Originally Posted by bpowa View Post
i think that chart is wrong.. I have 3.73 gear with 32' and I am cruising at 1900-2000 rpm at 65mph.
Read it a little more carefully, RPMs are given for a final drive ratio of 1.0:1, not what a transmission that has an overdrive final drive ratio would give.

That chart's RPMs would match my new 3-speed automatic's RPMs since it does not have overdrive, but it would not match my previous 5-speed transmission if I was in 5th gear... but it would if I downshifted to 4th gear which is 1.0:1.
__________________
When you have a choice, buy American.

Jerry Bransford is offline   Quote Quick Reply
Old 03-18-2008, 05:23 PM   #6
Jeeper
 
bpowa's Avatar
 
Join Date: Jul 2007
Posts: 836
I have a 4spd, so for optimal mpg on the freeway I should not use overdrive?
bpowa is offline   Quote Quick Reply
Old 03-18-2008, 06:16 PM   #7
Knows a couple things...

WF Supporting Member
 
Jerry Bransford's Avatar
 
Join Date: Nov 2005
Location: Escondido, Calif.
Posts: 30,852
Images: 2
Quote:
Originally Posted by bpowa View Post
I have a 4spd, so for optimal mpg on the freeway I should not use overdrive?
No, you should use overdrive for best fuel economy. Where did you get that you should not? Re-read what I said, I only said that your RPMs are while you're in overdrive, that chart is showing RPMs without the use of overdrive so they will be higher than your RPMs.
__________________
When you have a choice, buy American.

Jerry Bransford is offline   Quote Quick Reply
Old 03-18-2008, 10:03 PM   #8
Jeeper
 
JCS05Rubi's Avatar
 
Join Date: Mar 2007
Location: central md
Posts: 2,748
that chart is showing the gear ratios for direct drive, not over drive. If you have a 3spd auto, the chart is correct for 3rd gear.

4spd auto, the chart will show your rpms in 3rd gear, your 4th gear rpms will be lower based on your overdrive ratio in the trans.

5spd manual the chart will show your 4th gear rpms, 5th will be dependant on ratio

6spd manual the chart will show your 5th gear rpms, 6th will be dependandt on ratio..

etc. etc.

__________________
Dad's Blog! http://justkeepliving.blog.com/
T-Shirts for Dad, based on his "Just Keep Living" philosophy. http://www.cafepress.com/justkeepliving


1997 Wrangler Sport - 5 spd. Retired
2005 Wrangler Rubicon - 6spd|KC 55watt fogs, IPF H4s with fatboys,Uniden CB radio/Teraflex mount
http://www.4x4s-pod.com/
JCS05Rubi is offline   Quote Quick Reply
Reply

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Jeep Wrangler 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:
If you do not want to register, fill this field only and the name will be used as user name for your post.
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:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools

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 Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
33' Tires Gear Speedo Help. Giancarlo TJ Tech Forum 2 09-08-2008 04:46 PM
Gear ratio advice for 33" tires on 4 Cylinder BillD TJ General Discussion Forum 19 02-29-2008 10:18 AM
(4) factory YJ steel wheels and tires - Alabama cherokee980 Classifieds Archive 1 02-01-2008 09:58 AM
(5) factory '05 TJ tires and wheels - Alabama cherokee980 Classifieds Archive 3 01-09-2008 03:07 PM
Trans gear ratio's "W4MAC" TJ General Discussion Forum 10 08-25-2007 07:56 PM


» Rates
Get low rates on auto insurance in Canada!

» Network Links
»Jeep Parts
» Featured Product

All times are GMT -5. The time now is 03:43 AM.



Jeepģ, Wrangler, Liberty, Wagoneer, Cherokee, and Grand Cherokee are copyrighted and trademarked to Chrysler Motors LLC.
Wranglerforum.com is not in any way associated with the Chrysler Motors LLC