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Unread 05-21-2010, 01:38 PM   #1
carcam
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Please help me get to 70 mph on the Hwy-

or at least keep me there.

Just put a 2.5" RC lift and 35" BFG AT's on a 2010 Rubicon. It looks amazing and rides surprisingly well. I have a 3 hr hwy drive I have to do occasionally and it's no problem at 65mph- speedo reads 60 but it's actually 65. But at 70mph, it's a struggle and wants to downshift at the slightest headwind or small hill. At 65, I'm getting 16.5 mpg but at 70 it drops to 14.

I knew the lift and bigger tires would have trade-offs and expected some of this. It's actually better than I thought around town. But I would sure like to be able to cruise just a little bit faster on the highway without the big drop in mpg or constant downshifting.

Any ideas? I tried a Superflow muffler but had to take it off- not a pleasant sound, drone at 2k rpm and the reduced back-pressure decreased the low end torque so it was noticeable slower around town.

I'm thinking I may have to change the gear ratio from 4.10 to 4.56's. That would take care of the gear ratio difference but may not help with the extra weight of the 35" tires which must be a big strain on the little 3.8L. Anyone tried this- couldn't find it in a search. The adds for Superchip and maybe a cold air intake sound like a possible solution to pick up enough HP to get me to 70 comfortably but I've heard mixed results and there's the hassle of re-flashing when taking it back to the dealer for warranty work.

Short of an AEV Hemi conversion, I may be stuck in the slow lane forever unless you have some ideas. Thanks for any thoughts.

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Unread 05-21-2010, 01:53 PM   #2
toyota4x405
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Put in the new gears and you won't have any problem. Cold air won;t do anything. It is hard to get any HP out the minivan engines. Just change gears and go.
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Unread 05-21-2010, 02:02 PM   #3
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New gears and program computer so the shift points and tire size are correct.
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Unread 05-21-2010, 02:18 PM   #4
owtdorz
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How many miles are on it?
I have a 4dr Rubi 35s and loaded down and have no problems reaching 85-90.
No performance mods!
Drive it like you stole it so the computer adjusts to driving style and is past 2000-2500 miles.
The PCM is detuned until then.
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Unread 05-21-2010, 02:22 PM   #5
RMiksits
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you have 5.13 gears that helps a lot bud.
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Unread 05-21-2010, 02:25 PM   #6
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you have 5.13 gears that helps a lot bud.
They are still in my garage.
I am still running 4:10s
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Unread 05-21-2010, 03:12 PM   #7
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carcam:

Expensive but Perfect Solution - Regear to 5.13-5.38 + AEV ProCal to calibrate shift points and gear ratio
Cheaper but Less Effective Solution - AEV ProCal to calibrate shift points + occasional use of the O/D off button



I've personally run 35"s with 4.10s and an automatic transmission (plus hundreds of pounds of add-ons) for over 35,000 miles now and I'm only planning on regearing if $1200 bucks lands in my lap. Granted I live in FL where there isn't a whole lot of hills but the hills I do drive (e.g. bridges) are overcome by turning off OverDrive.
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Unread 05-21-2010, 03:23 PM   #8
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Sounds like some new gears are in my future and maybe a computer re-program. Makes more sense than engine mods (on a minivan engine? never heard that but makes sense). Thanks a lot for the tips!
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Unread 05-21-2010, 03:30 PM   #9
Colockum
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Get on a looong down hill...

Don't go with the Cold Air Intakes, the really don't do anything especally for the price. Check out this thread http://www.jeepforum.com/forum/f9/fa...intake-558437/
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Unread 05-21-2010, 03:32 PM   #10
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I know your pain. My X had 4.10's, and I ran 35's with the 4.10's for a year. It wasn't bad around town. But on the highway at 70mph, it was in and out of OD so much it was annoying. Any little incline or wind and it had to drop out of OD. And the cruise control was completely useless.

I have 5.13's now, and it is much better. It will hold 70+ with cruise control no problem, even on most hills.
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His: '07 Unlimited X, Jeep Green, Hardtop, auto, 5.13, 2.5" RC lift and too many mods to list

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Unread 05-21-2010, 03:40 PM   #11
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N2 Rock- why 5.13's and not 4.56's which is what would take it back to stock?
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Unread 05-21-2010, 03:56 PM   #12
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Honestly- I felt the 4.10 with stock 32's was barely adequate. Even with the stock setup, it still had a lot of downshifting. Going up to 4.56 is such a small change, it wouldn't be worth the expense. I had a friend that went to 4.88's and he felt it still was not enough gear.

In the various JK forums, the general consensus amongst all owners that have regeared, is that 4.88 for manual, and 5.13's for auto is ideal for running 35" tires.

So with all that, I opted to go with 5.13's. And the performance is where I feel it should be. Gas mileage is the only negative. I get the same mileage around town, and about 2-3mpg less (than stock) on the highway.
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His: '07 Unlimited X, Jeep Green, Hardtop, auto, 5.13, 2.5" RC lift and too many mods to list

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Unread 05-21-2010, 05:51 PM   #13
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You need to re-gear. There is no debate. As soon as you added larger tires you also changed you your final drive ratio. If I were you I would listen to what the vast majority on this forum and the other forum are saying and go with 4:88’s for your manual trans.
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Unread 05-22-2010, 09:49 AM   #14
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Re-gear it is- thanks for all the advice!
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Unread 05-22-2010, 03:53 PM   #15
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OK- so I'm going to put in 5.13's. Is this something I can do myself? How involved is it- special lubes, gaskets, etc. I'm assuming it has to be front and back. It's got to be easier than the lift with spring compressors. I have a lift, tools, some basic mechanical ability- not like most of you (day job is a root canal guy). Should I send it out? Do like getting my hands greasy (when it turns out). Any advice is appreciated.
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