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Unread 03-14-2010, 04:03 PM   #1
mclaggan123
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gearing performance question

I have a 07 unlimited automatic with 35's and im thinking of regearing to 5.13's. The question i have is for the people that have this setup how is the performance of the jk now? What is the highway driving like (speed, passing, mpg). Guys in my jeep club say its too much gear and that ill lose alot of highway speed. None of them have jk's and are basing it off of tj's and yj's. They think i should go with 4.88's. Any help would be appreciated.

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Unread 03-14-2010, 04:54 PM   #2
RockyClymer
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More info - why do you want 5.13 or any other ratio for that matter? Bigger tires? More info on what you are trying achieve in the end would be helpful to all here. Your profile says JK and a rear with a 4.10 is that still correct?
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Unread 03-14-2010, 06:08 PM   #3
asknight
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Quote:
Originally Posted by RockyClymer View Post
More info - why do you want 5.13 or any other ratio for that matter? Bigger tires? More info on what you are trying achieve in the end would be helpful to all here. Your profile says JK and a rear with a 4.10 is that still correct?
Perhaps reading his post would help. He has 35's. He's trying to achieve good driveability on the highway, safe passing of vehicles, etc.

5.13 is the ideal ratio for an automatic trans JK to do that.
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Unread 03-14-2010, 06:26 PM   #4
mclaggan123
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yes i still have 4.10's and with the 35's its a little sluggish. im looking for better low end for the trail and still have plenty of high end for the highway. people say its better after they regear but they never give details as to how its better. do they max out at 55 or can they do 70, i want to know things like that. thanks
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Unread 03-14-2010, 07:28 PM   #5
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I have 35's with 5.13 on a 07 automatic JK and it does great. As stated in prior posts, I drive daily on the interstate and average 70 mph without any issues. I am able to pass comfortably on the highway but this is no sports car. As for gas mileage I did not do so well until I invested in the max energy programer. Before I was getting 13-14 mph and now with the programmer I have actually been averaging 16-18 mph on the highway and a little more power. I have a friend who has 4.88 with 35's and he now wishes that he would of gone with 5.13's. My opinion is to go with 5.13 with the automatic but I am also not a mechanic by any means. Most of my jeep buddies recommended that I go from 4.10 to 5.13 just in case I ever wanted to go with 37" tires which I have been thinking about. Hope this helps!
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Unread 03-15-2010, 10:02 AM   #6
Xtremjeepn
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I have 5:13 and the auto on 35s and almost wish I had gone deeper. The OD on the auto is 0.69 which is WAY too deep for the motor.

The 6 speed manual does not have as deep of an OD (0.84 vs the auto 0.69)and also has 2 more gears before the OD.

At highway speeds an auto with 5:13/35s is only turning 142rpm LESS than as STOCK Rubi 6 speed (4:10/32s)



Edit:


Here is the math.

Auto with 35s and 5:13 at 65mph =2209 rpm in OD.
Stock 6 speed with 32s and 4:10 at 65 =2351 in OD.



A stock Rubi auto turns 1931rpm at 65mph which is WAY WAY too low for the motor to spin and have any power. This is why the auto is always coming out of OD on the higway. The 3.8L is just not turing enough power at that RPM. That is nearly 400rpm less than the stick stock!!!
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Unread 03-15-2010, 10:33 AM   #7
asknight
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Quote:
Originally Posted by Xtremjeepn View Post
I have 5:13 and the auto on 35s and almost wish I had gone deeper. The OD on the auto is 0.69 which is WAY too deep for the motor.

The 6 speed manual does not have as deep of an OD (0.84 vs the auto 0.69)and also has 2 more gears before the OD.

At highway speeds an auto with 5:13/35s is only turning 142rpm LESS than as STOCK Rubi 6 speed (4:10/32s)



Edit:


Here is the math.

Auto with 35s and 5:13 at 65mph =2209 rpm in OD.
Stock 6 speed with 32s and 4:10 at 65 =2351 in OD.



A stock Rubi auto turns 1931rpm at 65mph which is WAY WAY too low for the motor to spin and have any power. This is why the auto is always coming out of OD on the higway. The 3.8L is just not turing enough power at that RPM. That is nearly 400rpm less than the stick stock!!!
You forgot a couple, for perspective.

42RLE auto with 32s and 3.21 (stock) at 65mph = 1512 rpm in OD
42RLE auto with 32s and 3.73 (stock) at 65mph = 1757 rpm in OD.

So, the 42RLE auto with 5.13s and 35s gives us an increase of 500-700 rpm over the stock setup of most models. You quoting the Rubicon gearing with a manual trans just muddied the waters even more.

I would like to retain the ability to just turn off OD when I need to, rather than throw all fuel economy out the window by cranking 3K+ rpm at 65 mph.
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Unread 03-15-2010, 10:33 AM   #8
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I've got the 5.13/auto on 35's. It drives like it did stock really... and that's what the regear is supposed to do. It's not super torqued or whatever.. you'd have to go deeper for that, like what was said, but the 5.13 on an auto just brings the ratio back close to stock ( with 35's ).

I flashed mine, and that has added some perks , but it's still not a V-8. It's a Jeep and unless you re-motor them, they haven't had awesome power since the CJ7-s from the 80's. If you're thinking of the money though... if you regear, you'll need to flash it with something so the computer knows what you did. Good luck.
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Unread 03-15-2010, 10:52 AM   #9
Xtremjeepn
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Quote:
Originally Posted by asknight View Post
You forgot a couple, for perspective.

42RLE auto with 32s and 3.21 (stock) at 65mph = 1512 rpm in OD
42RLE auto with 32s and 3.73 (stock) at 65mph = 1757 rpm in OD.

So, the 42RLE auto with 5.13s and 35s gives us an increase of 500-700 rpm over the stock setup of most models. You quoting the Rubicon gearing with a manual trans just muddied the waters even more.

I would like to retain the ability to just turn off OD when I need to, rather than throw all fuel economy out the window by cranking 3K+ rpm at 65 mph.
I didn't forget them, I left them out because they were not relevant to me and I did not want to do the math for ever possible combo, just the most relevant ones.

More RPM does not immediately translate to fuel consumption. Running an engine in it's efficient power range takes less fuel than lugging it at too low of an RPM or too high.

I increased my average mpg by about 2-3mpg going from 4:10 to 5:13 with 35s. The engine is MUCH happier at the increased rpm. (and where in the numbers posted above do you see ANY of the Jeeps running 3K rpm at 65 mph? The highest is the stock 6 speed rubi at 2300)
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Unread 03-15-2010, 10:55 AM   #10
Xtremjeepn
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Quote:
Originally Posted by yippie21 View Post
.... they haven't had awesome power since the CJ7-s from the 80's. .

What "Awesome" power are you talking about? The 4.2L put out 112hp and the V8 put out 160hp The 3.8L is litterally the most powerful stock motor EVER offered in a convertable Jeep.
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Unread 03-15-2010, 11:56 AM   #11
mclaggan123
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thanks guys thats the kind of stuff i wanted to know. i already have the flashpaq for the tire size so regearing ill be able to just reprogram it again.
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