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Unread 05-02-2013, 09:37 PM   #1
anjum69
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2011 JK Wrangler 
 
Join Date: Feb 2013
Location: Portage, MI
Posts: 22
Bigger tire and Gear Ratio question.. Please help

Dear All Jeap owners,

I'm new to this.. I have 2011 Jeep JK unlimited 4 door Auto. Just replaced stock tires and wheels(255/70R18) to 315/70R17 and new ATX wheels size 17". Still have Stock Gears (3.73). Please help me to understand this.. Do I need to regear and go smaller.
After fixing my tire size using Superchip flashcal to 34.50" and my speedometer is showing correct speed.
I noticed the current Gas millage is 14 miles/Gal in City. I didnt try the Highway drive yet.
Today I took Jeep on highway and using cruise my RPM on 70MPH was showing around 1800 RPM, but after driving few miles may be 7-8 miles the RPM increased to 3,000 ?? Is it called down shifting? If yes then why after 5-10 miles .

While increasing speed to 70 MPH the rpm increases to 3,000 RPM and the stable at 1800 for around 8 miles before it goes back to 3,000 RPM.

Please explain me about this behavior.

second do you think if I go for re gearing, Jeep will give me better Millage ? in City and Highway both ? is 14MPG bad in city after the bigger wheels?

What should be a good gas millage with this setup on Highway?

Thanks in Advance..

Note: I have already purchased a Yukon 4.88 Gears but didn't install yet .. waiting for your advice..

Anjum

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Last edited by anjum69; 05-03-2013 at 09:41 AM..
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Unread 05-02-2013, 10:04 PM   #2
222Doc
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2012 JK Wrangler 
 
Join Date: Sep 2011
Location: Lake Havasu City, AZ
Posts: 4,466
30,000 rpms????? not possible. You might want to get an after market trans cooler too. larger tire means more heat in the trans. they are know to over heat the trans. Few here have pics of what can happen, fire.

regardless, sounds like you have a Auto. It needs to be regeared. 5:13 at least. will help a bit do not expect much. couple more MPG. but it wont jump back and forth from overdrive to 3rd.
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Unread 05-03-2013, 09:42 AM   #3
anjum69
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2011 JK Wrangler 
 
Join Date: Feb 2013
Location: Portage, MI
Posts: 22
Oops sorry . not 30,000 I mean 3000 RPM .. Do you think 4.88 will work too? I have heard 5.13 will take more Gas..
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Unread 05-03-2013, 09:42 AM   #4
anjum69
Registered User
2011 JK Wrangler 
 
Join Date: Feb 2013
Location: Portage, MI
Posts: 22
Updated.. the Info

Quote:
Originally Posted by 222Doc View Post
30,000 rpms????? not possible. You might want to get an after market trans cooler too. larger tire means more heat in the trans. they are know to over heat the trans. Few here have pics of what can happen, fire.

regardless, sounds like you have a Auto. It needs to be regeared. 5:13 at least. will help a bit do not expect much. couple more MPG. but it wont jump back and forth from overdrive to 3rd.


Oops sorry . not 30,000 I mean 3000 RPM .. Do you think 4.88 will work too? I have heard 5.13 will take more Gas..
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