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Old 01-25-2020, 03:09 PM
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Trans and gears

Hello Guys
I have a 2014 jku Freedom Ed. with automatic and original 3.73 gears. I have since changed 35" tires with 4.56 gears with air lockers. By the charts my performance should be the same as stock. Now at 70 mph my trans will down shift way more than it did stock pulling a hill or even head wind. At 55 mph I can get 16.3 mpg but at 70mph 12 mpg. I have the speedo recaled to correct speed tire and gear. Just seems I'm turning to much rpm at 70mph 2800-2900. It feels like the trans could hold a little longer in the higher gears before down shifting. So II guess I'm asking is this normal? I know I'm asking a lot out of the 3.6 at interstate speeds now. Would a tune help with the power and shift points?
Thanks

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Old 01-25-2020, 03:40 PM   #2
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The weight of the tires is more demanding of the engine even with the gears. That's why top speed goes down in relation to gear/tire size. To.maintain speed you.must maintain gas

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Old 01-25-2020, 05:58 PM   #3
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^ this. Regearing puts your engine back into the range of PRM the Jeep was designed for, but the Jeep is heavier with bigger tires and has more inertia, plus the tires have more rotational inertia. It takes more power to accelerate - and harder braking to slow down.
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Old 01-25-2020, 06:23 PM   #4
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Also the 3.6 pentastar loves to rev. It is made for it. I learned all this stuff from the guys on the jk tech forum. Lots of knowledge there

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Old 01-26-2020, 12:45 PM
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Well that's what I was thinking.
What about a tuner, would it be worth it?
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Old 01-26-2020, 12:47 PM   #6
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Get a super chips flashcal and play with the tire size plus or minus one size either way. Everyone has different opinion on how they want their ride.

Oh, you didnt say If you flashed the computer with the new gear/tire size, you will need to do that.... necessary to order the super chips.

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Old 01-27-2020, 03:25 AM
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Have the flash cal, thinking on converting to flash pac. Didn't know if the conversion is worth it.
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Old 01-27-2020, 08:46 AM   #8
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What tires?

IMO...4.56 is too much gear for 35s on a daily driver unless you live in a particularly mountainous area. I use 4.56 with 37s for my DD.

You should set the tire size using the result from this formula which requires find the revolutions per mile for your tire.

63,360/revs per mile = X
X/3.14159 = rolling tire height. Use this number in your programmer.
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Old 01-27-2020, 01:21 PM   #9
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My sister in-law has a JK with auto trans, 35's, and 4.56 gears. It works really well. Even though her "35 inch tires" are more like 33.5" tires (BFG KO2's). Around here isn't as flat as, say, Florida. But it isn't really hilly either, especially where she lives.
The first question I would ask is are you sure you have the right tire size programmed in to set the correction for the new size tires. You can't go by the published tire size. You need to measure.
Even if the gearing is the same overall with the new tires and new gears as mentioned you will not get as good a performance or mpg. The new tires are bigger and heavier. The take more power to spin, more power to stop, and have more rolling resistance.
I don't have a lot of faith in magic tunes unleashing better performance in your engine, but it is an option. But I am doubtful that it will magically get your mpg back.
Jeeps aren't aerodynamic. The faster you push them the worse your mpg.
And our Jeep pulls around 3,000 rpm cruising down the highway. That isn't even breaking a sweat for the 3.6.
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Old 01-27-2020, 02:21 PM   #10
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Quote:
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What tires?

IMO...4.56 is too much gear for 35s on a daily driver unless you live in a particularly mountainous area. I use 4.56 with 37s for my DD.

You should set the tire size using the result from this formula which requires find the revolutions per mile for your tire.

63,360/revs per mile = X
X/3.14159 = rolling tire height. Use this number in your programmer.
Everyone has different opinions on "optimal gearing." I think 4.56 is hugely undergeared for 37s. I use 5.38s with 37s.

Agree with getting the proper number in the programmer. @JRPalmer what did you enter into the programmer? Verified by some sort of external GPS?

The only "tuner" i would consider is something like the pedal commander that adjusts the throttle in relation to the pedal position. I dont think the 3.6 will benefit much from any performance engine tuning.
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Old 01-28-2020, 12:23 AM
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Well guy's thanks for input. I have tire size as 33.75, its off by 1/10th at 75. IE reads 75 speedo/74.9 gps. Did upgrade to flashpac today. Put 87 octane fuel tune in. I love the shift points, holds in OD pulling hill that it formally down shifted to 3rd. passing from 40 mph to 65 was easy. I have power now, like or better than before. I live 20 miles from a real town, in and out today was way better. Will see what this tune does for mpg after i get normalized with it. I had loony tunes in my 6.0 power stroke ford, that still passed faster. But dyno said 535 hp with my injectors and turbo
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Old 01-28-2020, 09:28 AM   #12
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Also the 3.6 pentastar loves to rev. It is made for it. I learned all this stuff from the guys on the jk tech forum. Lots of knowledge there

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Made for revving?
If it was made for revving, they should have put better quality cam followers in it. They can't keep the rollers in 'em not revving them
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Old 01-28-2020, 11:00 PM   #13
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Made for revving?

If it was made for revving, they should have put better quality cam followers in it. They can't keep the rollers in 'em not revving them
I have only done lifters and injectors inside the top end. I'm more into differential work and suspension, but as I understand the 3.6 pentastar is a mercedes engine? And it was supposed to raise shift points to higher revs? I'd have to look to find the link

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Old 01-29-2020, 04:38 PM   #14
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Naa, it's not a Mercedes, however it was built when it was Daimler Chrysler, it shares basic design with the M276 Benz engine.
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Old 01-29-2020, 04:50 PM   #15
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Naa, it's not a Mercedes, however it was built when it was Daimler Chrysler, it shares basic design with the M276 Benz engine.
Perhaps they are confusing it with the NAG1/WA580.
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Old 01-29-2020, 05:10 PM   #16
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Perhaps they are confusing it with the NAG1/WA580.
That could be

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