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 Post subject: Has anyone swapped 3.55's into a crd
PostPosted: Wed Jan 16, 2019 1:07 am 
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I am thinking of buying a set of 3.55 axles from a manual trans 3.7 and swapping them into my wife's crd, we use it for road trips and it does not tow, that's what mine or the trucks are for. That said swapping from 3.73 to 3.55 would drop the rpm to about 1927 @70 vs 2020 @70 so it should have decent mpg improvements when cruising. I was wondering if anyone has done this change before and what were the results.


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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Wed Jan 16, 2019 12:36 pm 
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My66dodge wrote:
I am thinking of buying a set of 3.55 axles from a manual trans 3.7 and swapping them into my wife's crd, we use it for road trips and it does not tow, that's what mine or the trucks are for. That said swapping from 3.73 to 3.55 would drop the rpm to about 1927 @70 vs 2020 @70 so it should have decent mpg improvements when cruising. I was wondering if anyone has done this change before and what were the results.


I think maybe one of the guys around here who had done a manual swap may have gone with 3.55's.

I think its a good idea personally. Considering the torque these make, I don't understand why they have such short gearing and why they turn so many RPM on the freeway. I ran 31" Wrangler Duratracs on 3.73's, which would be about equal to running stock size tire on 3.55's, and it pulled it along just fine. I am now on 32" Duratracs and swapped in 4.10's and kinda regret it.

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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Wed Jan 16, 2019 12:54 pm 
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X2, I love the ratio with the 245/75/16 but it does increase the transmission temp a lot creeping up long grades offroad going really slow.

I'm pretty sure the manual swap guy had to have custom gears made for the front dif though...

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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Wed Jan 16, 2019 1:49 pm 
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Quote:
I ran 31" Wrangler Duratracs on 3.73's, which would be about equal to running stock size tire on 3.55's, and it pulled it along just fine. I am now on 32" Duratracs and swapped in 4.10's and kinda regret it.

This is the best answer.

I've run 215-85-16 tires. . EDIT: 30.4" diameter :oops:
No lift and no rubbing. Didn't even have to pound down the pinch welds.
Worked great for highway use.

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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Thu Jan 17, 2019 3:19 pm 
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flash7210 wrote:
Quote:
I ran 31" Wrangler Duratracs on 3.73's, which would be about equal to running stock size tire on 3.55's, and it pulled it along just fine. I am now on 32" Duratracs and swapped in 4.10's and kinda regret it.

This is the best answer.

I've run 215-85-16 tires. . EDIT: 30.4" diameter :oops:
No lift and no rubbing. Didn't even have to pound down the pinch welds.
Worked great for highway use.


I really don't know why so many people insisted 4.10's were the greatest thing ever. Unless you are stock or are running the neutered Eco-tune, I can't imagine the Jeep can't pull along in 5th gear no problem with 3.73's on any size tire you can fit.

off-road might be a different story, but I don't think the OP is concerned about that.

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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Mon Jan 21, 2019 10:18 am 
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I understand the reasons not to do the 4.11s I did the 4.11 in dads rig, then I swapped it back to the stock axles. I will keep the 4.11 axles for my build since I am planning a JBA 4 inch and 265/75/16's with rubicon wheels. Since my wife's rig is the freeway cruiser I want to build it to get the best highway mpg I can, currently on the evic it does 20-22 in town and 28-36 freeway with the Eco trans tune and suncoast converter and both weeks kits, I look forward to the full tq tune improvements in mpg. I plan to install eaton true tracs in both axles before I put them in. My wife doesn't go off road but the last thing I want it for her to get stuck in a snow bank, this way the only way she can get stuck is by getting high centered in the snow. I found the 3.55 axles for under 600 shipped to my door by a local yard


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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Tue Jan 22, 2019 7:01 pm 
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My66dodge wrote:
I understand the reasons not to do the 4.11s I did the 4.11 in dads rig, then I swapped it back to the stock axles. I will keep the 4.11 axles for my build since I am planning a JBA 4 inch and 265/75/16's with rubicon wheels. Since my wife's rig is the freeway cruiser I want to build it to get the best highway mpg I can, currently on the evic it does 20-22 in town and 28-36 freeway with the Eco trans tune and suncoast converter and both weeks kits, I look forward to the full tq tune improvements in mpg. I plan to install eaton true tracs in both axles before I put them in. My wife doesn't go off road but the last thing I want it for her to get stuck in a snow bank, this way the only way she can get stuck is by getting high centered in the snow. I found the 3.55 axles for under 600 shipped to my door by a local yard


I've done front and rear truetrac's on mine and love it so far. with the 265/75R16 Duratracs and full-time 4x4, its an excellent snow rig.

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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Wed Jan 23, 2019 8:56 am 
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just a note:
DTTs for 3.55 are different than for 3.73/4.10, so if you decide you don't like 3.55 you're tossing them with the DTTs and starting over.

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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Wed Jan 23, 2019 10:10 am 
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Good point, thanks, I will swap the axles then add the dtt's later after I give yalls a review of how the 3.55's perform


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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Tue Aug 27, 2019 8:58 pm 
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Update, I am putting the 355’s in next weekend, I will drive it for a month or so and report back


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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Wed Aug 28, 2019 3:29 pm 
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I think it will do fine. Since most drive in overdrive all the time anyway worse is take OD off and run little higher rpm than you do now.

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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Wed Aug 28, 2019 4:28 pm 
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Honestly I think you could have gone a lot taller - I don't know if there is something in the 3.2x range, but only dropping the RPM by ~100 doesn't seem like enough of a change. Each gear shift is good for around 400-700 rpm, if you wanted to "add another gear" you'd need a much bigger change and the engine CERTAINLY has the torque to do it.

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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Thu Aug 29, 2019 9:49 am 
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It only cost me 50 shipped to my work for the set of axles :) from an 06 manual v6 so almost free other than time. The best mileage is about 1850 rpm according to Keith, and at 70 we are turning 2050 or so, so dropping to 1920 if I remember correctly should be noticed.


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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Thu Aug 29, 2019 5:41 pm 
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My66dodge wrote:
It only cost me 50 shipped to my work for the set of axles :) from an 06 manual v6 so almost free other than time. The best mileage is about 1850 rpm according to Keith, and at 70 we are turning 2050 or so, so dropping to 1920 if I remember correctly should be noticed.
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Please do as scientific a test as possible - fuel right to the lip, run at least a 50 mile loop (100 mile round trip) at as steady highway speed as possible - and then fuel right to the lip again. Then after the upgrade, do the same route again as close as possible to all the same conditions to minimize statistical noise... And see what the mileage change is.

If it is an easy swap and shows promising results, you may have landed on an upgrade that a lot of us will want to do! I'd certainly be interested, knowing that these things can easily tow 8,000 lbs, I'll happily give that up and limit towing to 5000 if I can get the mileage up 5-6 mpg when not towing.

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 Post subject: Has anyone swapped 3.55's into a crd
PostPosted: Thu Aug 29, 2019 10:45 pm 
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Will do, the best way to prove is to daily drive for a week 20 min to work, I do the same drive every day so I know how it does, on my evic I am 26.5 - 27.5, always so I will easily see changes. As far as ease of swap, it’s cake to swap axles, last year I put 4.11 axles in mine and swapped it back, so it’s about a day job for most guys in the driveway (a little easier owning a shop)


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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Thu Aug 29, 2019 10:54 pm 
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My66dodge wrote:
Will do, the best way to prove is to daily drive for a week 20 min to work, I do the same drive every day so I know how it does, on my evic I am 26.5 - 27.5, always so I will easily see changes.


Agreed that you'll notice any changes on the EVIC, but I'd recommend using Geordi's method as well. The EVIC uses the tyre size, revs per mile, and axle ratio numbers (amongst other things) stored in the BCM as part of its fuel economy calculations - and in order to find out what those numbers are, you'll need a DRB-III to pull them. Basically, if you don't know what the BCM is passing to the EVIC, there's not really a good way to tell how much built-in inaccuracy may be in the EVIC vs. how much it's also being thrown off by the change in tyre or gear size.

I understand where you're coming from: I watch my EVIC as well since I know where the economy averages it reports usually fall with my driving patterns. But for figuring out actual mileage, I always hand-calculate it. There's just too much sway in the EVIC's numbers to trust them.

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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Fri Aug 30, 2019 8:33 am 
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Unfortunately the drb3 won’t help... already tried, on the 06 the abs is CAN so it is a starscan vehicle, I wanted to buy one for the shop but I can’t justify the price since it’s use is limited... the dealer has a real starscan so I had them try to reprogram the tire size and they couldn’t (I know the tech doing the work) so I had Keith take a crack at it... I got a few more Chrysler people involved ( like a trainer) the only way to change the Speedo on this one is with the regional level programming device... or use a pulse divider on each wheel speed sensor to speed them up or slow them down...


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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Fri Aug 30, 2019 10:06 am 
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I believe you will probably gain about 2 mpg maybe 3 tops but remember you do not have to change both axels at first. You could just change rear axel first and drive it a week or two to get mileage data etc. Then if you decide to stick with it you could then commit to true tracs if justified and do the front in all at one time rather than piece meal. I think the 3.5 will be ok because if you want you can still change to 31 inch or so tires without lift and really maximize mileage and this could be fine tuned fairly easily. Just get a pair of used tires for rear and testing purposes in whatever size and try them a couple of weeks then when you settle on optimum size get that size all around. Of course, make sure they will fit on front without lift first. I believe you can probably gain another 1-2 mpg by changing tire size from stock also. You may have to keep top speeds at 70 or below to gain much mpg though. I think you can gain 3-4 mpg total but would be great if 4 or more. I get my best mileage with the 32 inch tires at about 1750 - 1800 rpm.

Good Luck! I will be following your results with great interest!

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 Post subject: Has anyone swapped 3.55's into a crd
PostPosted: Fri Aug 30, 2019 9:35 pm 
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I got the rear axle today but they couldn’t find the front they promised so they are sending me one Tuesday, I am swapping both axles to get a real long term result, between December and May I make about 10 trips from Kennewick to Seattle or Portland, every year for my sons gymnastics competitions so is there is a difference I will see it, average over snowqualimie is about 34 -36 depending on weather, our best is 37... I run 245-70-16’s on my wife’s rig, I run the slalom KSI winter tires in the winter. If I like the re gear I will add truetrac’s, I am betting on 3-5mpg, and with the cost of the axles and the fluid to service them and the new axle seals and bearings for the front I am still at less than $100 into the project , so if I only get 1-2 it’s still a great win. Imagine the mileage you get at 65, but instead you’re doing 70, from my quick math it will put me at 38.5-40


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 Post subject: Re: Has anyone swapped 3.55's into a crd
PostPosted: Sat Aug 31, 2019 3:33 am 
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The EVIC lies to you by 3+mpg, and with larger tires that error is compounded. You really need to hand-calculate to get an accurate number. I'd LOVE to see a 40mpg CRD, but I think a lot of your numbers is mileage going downhill. ;)

Now if someone in Indiana or Louisiana can match those results on dead-flat land... Then we are really talking!

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