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Discussion Starter #1
Just bought a 08 jk 4door automatic. 2.5 lift, 35 kenda m/t on eagle alloy. Mostly highway driving.
After buying I realize when speedo says 60 I'm doing 70mph. Always seems like I'm giving it gas and boy does it go quick! Also noticed at "70" it's running at about 4500 rpm.
Was told several things, looking for some clarification.
1 told to recalibrate. $200-250
2 told to regear to 513 $2500
Seems to be a lot of difference in opinions but why?
Like I said its my everyday ride, a lot of highway miles.
 

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I have an 08 with "35"s. And 5.13s . I also have the speedo calibrated with a superchips flash paq. I'm running about 2700 at 70mph..

You may want to try just clicking off the over drive to see if you jku is even using it. By the numbers you may have gears already. It would seem odd though that PO would put in gears and not correct the speedo.

You should get some sort of tuner to fix the speedo issue and also fix the tpms tire psi limits. If you have a max tow pkg it may already have 3.73 gears. I'd still suggest 5.13s as it made my jeep much more highway friendly in the hilly area I live in..
 

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Discussion Starter #5
Thanks for the reply norville. I'm not sure what's been or not been done to it.
I'm getting conflicting options as to what to do. One says 513 and reprogram, one says you can't run 513 on highway, another says NO need to regear Factory is fine with 35's just recal speedo and shift points.
So confused as to what's the correct thing to do.
Was also told if I don't regear and recal. I will burn the trans up. Also advised installing a secondary trans cooler.
Not sure if people think I have a no limit credit card or they just being honest. I'm a dumb firefighter not a mechanic.
 

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Thanks for the reply norville. I'm not sure what's been or not been done to it. I'm getting conflicting options as to what to do. One says 513 and reprogram, one says you can't run 513 on highway, another says NO need to regear Factory is fine with 35's just recal speedo and shift points. So confused as to what's the correct thing to do. Was also told if I don't regear and recal. I will burn the trans up. Also advised installing a secondary trans cooler. Not sure if people think I have a no limit credit card or they just being honest. I'm a dumb firefighter not a mechanic.
For 35s , 5.13 for an 08 auto, 4.88 for manual trans. Tried and true. If you have an auto with 3.73 shutting the OD off will give you the equivalent final drive ratio of 5.13 so you can see how you like it.
 

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Just bought a 08 jk 4door automatic. 2.5 lift, 35 kenda m/t on eagle alloy. Mostly highway driving. After buying I realize when speedo says 60 I'm doing 70mph. Always seems like I'm giving it gas and boy does it go quick! Also noticed at "70" it's running at about 4500 rpm. Was told several things, looking for some clarification. 1 told to recalibrate. $200-250 2 told to regear to 513 $2500 Seems to be a lot of difference in opinions but why? Like I said its my everyday ride, a lot of highway miles.
Sounds like one is giving you advice on addressing the speedo calibration and the other is giving you advice on addressing the performance RPM and MPG issue. Neither are exclusive of the other.

JK on 35" MT's and lots of highway miles isn't a good combination.
 

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To the OP. If you are running 4500 RPM at 70 MPH then your Jeep has already been re-geared. Buy a Superchips tuner or flashpaq and recalibrate your speedo yourself.
 

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To the OP. If you are running 4500 RPM at 70 MPH then your Jeep has already been re-geared.
Not necessarily. It's possible the transmission isn't shifting out of 2nd gear. A computer that needs to be reprogrammed may do that.


Best bet is to figure out what gears are in there right now. Once you know that, the path forward becomes clearer.


Seems to be a lot of difference in opinions but why?
This is the internet. Everyone has an opinion, whether they're good or not.

BTW, I did a write up on why you want to change your axle gears when you change your tire size: http://www.grimmjeeper.com/regearing_basics.html

No matter what you do, getting the computer programmed correctly is key. If the computer thinks you're running different size tires and/or a different gear ratio it's not going to shift the transmission right. You absolutely need to get that done either way. You especially need to do that when you change gear ratio.

Once you get that sorted out, you can figure out if a gear change is something you need/want.
 

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By the rpm's, sounds like it's been regeared.
Jack the back wheels off the ground, put a mark on the tire, and see how many turns the driveshaft makes to turn the wheel once, that will give you a ball park idea of gear ratio.
Touch over 4 times would be 4:10, 5 and a little would be 5:13, ect.
Then grab a flashcal, or Procal, and adjust your tire size and gear ratio, that should fix you up.
 

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35s and 5.13 on 08 auto JK, 2500rpm at 70, recalibrated with superchips trialdash
traildash - $500 (can re calibrate for a lot cheaper)
regear -$1500
 

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If your engine is really turning 4500 rpm at 70 mph, that is not good - especially since you said you do mostly highway driving. Hard on the engine, and bad for fuel economy. Idealy you should be running below 3000 rpm at that speed.

Before doing any work, you should find your rear end gear ratio so you will know what you are starting with. Here is a youtube video showing one way to do that. It would be best to have a friend help you count revolutions. Note that the method is slightly different for open differentials vs limited slip (the video explains that).

http://www.youtube.com/watch?v=oG0iurCYujE&ebc=ANyPxKoWTVz0a3sF6RFpIC_u0521fK5aE2DeKK8z1djHaqgzjyj-Ty-0RUN0T1heSIc5Fc7Jemp6ijHP-Ev5dsoQUKoJ2sWOQA
 

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Discussion Starter #14
Thanks all for replying.
So I need to update I must have been on a bit of an incline when I read that it was 4500rpm at 70. After review it actually is around 2200rpm.
After reading JTPhoto post, I did turn the OD off and man, so much better. Didn't feel like I need to keep giving it gas to hold a speed. It held a constant speed effortlessly.
Derf, read your article thanks for posting that. So I guess the first thing to do is to find out what's in it. It sure seems from all the post that 5:13 is the way to go. Just have to find out if the carrier can hold that is what I'm told.
Thanks again to everyone that post. Please continue. I'm defiantly learning a lot about my jeep and maybe it's helping someone else.
 
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