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Old 04-12-2011, 04:46 PM   #1
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285/70/17 auto and what gears

I have 2K burning a hole in my pocket and planned on getting Eaton E-locker for my front axle. Well, I figured since I have the axle apart anyways to go ahead and get new gears in the process. I currently have 3.73 and they are not that bad especially at 70mph but I am getting a trailer for some upcoming trips. That said I thought that I might be better off getting new gears to help with the towing

I have been at the off road shop today and we have been going round and round on the gear issue. He says that 4.88 would just be too much with my tire size. Said if I had 35's then 4.88 would be the right choice but he wouldn't recommend no more then 4.56 or even 4.10 for my Jeep with 285's. He said I would hate the 4.88 at highway speeds and my RPM's would be way to high at 70mph and my gas mileage would take a huge hit.

The issue then arose that Yukon was the only good gears for DD. Said the rest were just too noisy. The problem is Yucon doesn't make 4.10 or 4.56 for D30 axles. The best I could get was 4.88. You can get 4.56 in Yucon's for the D44 but not the D30. But that is were we left it today.

What does everyone think about the gear issue?

Also, was wondering. Will the 2007 4.10's fit the 2010 axles?

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Old 04-12-2011, 04:51 PM   #2
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ok thats a 32 inch tire and from the tire charts vs rpm charts you should be turning around 2400 rpm's with 4.88 not to bad if you ask me , so as i get some cash im going 4.88

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Old 04-12-2011, 05:00 PM   #3
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Well, they are 32.7 but the chart is showing 2400. He said that he thought it would be more like 2600 so 2400 is a little better. I don't know if they would that bad but I am sure they will help with towing though. What has others found as a MPG issue?
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Old 04-12-2011, 05:07 PM   #4
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Don't you have an auto transmission?

According to the little tire size link I use (here) and the attached gear ratio chart for JKs, there's no way 4.88s would "just be too much." You might want to consider 5.13s, maybe even 5.38s.

MOPWR2U got all this stuff down so maybe he'll chime in.

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Originally Posted by rics1997 View Post
Well, they are 32.7 but the chart is showing 2400. He said that he thought it would be more like 2600 so 2400 is a little better.
Really? I'm no expert, but unless the attached chart is wrong it's hard to see how you'd get to 2600 with 4.88s on 32.7" tires. Your tires would have to be only 30" to hit 2600 RPMs with 4.88s at 70 mph.

Even 32s would only be at 2476. There's obviously some variation for actual height, but assuming your tires actually measure 32", you could get 5.13s put in and only be at 2603 RPMs at 70 mph.
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Old 04-12-2011, 05:15 PM   #5
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I wouldn't go any lower than 488's.... Chances are your next set of tires will be bigger and better...
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Old 04-12-2011, 05:45 PM   #6
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Why dont you get some better axle shafts while your at it for that front 30. I played with this idea myself but decided to just get a 44 for the front instead and not worry about breaking anything down the road. I should have bought a Rubicon for the axle and T-case but its too late now and once I get a good 44 I'll be about where I want to. I can get beefed up shafts through my store and there not a bad price. If you wheel that thing you mind as well get some better shafts. I got a few sources on the hunt for a banged up Rubi and some one will find one. Just a matter of time before these things start getting banged up. Theres lots of Rubi's on the road that are yuppies DD's and someone will crack one up sooner or later.
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Old 04-12-2011, 07:42 PM   #7
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Not planning on going over 33's, though I might go to 295/70's which are a little over 33's but no more then that, I doubt I will need stronger axles unless I do decide to go to 35's but that is among many reasons I don't like the idea of 35 and larger tires. Too much stress on a Jeep. Will probably add gussets to the D30 though.

Still hoping to hear from someone with 4.88 and 33's to comment on gas mileage and how they feel about highway RPM's
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Old 04-12-2011, 07:57 PM   #8
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Thats good thinking and thats why I asked. I will keep with 285's on 17's for now myself until I properly fix the other underlying things. What I do 35's will not make or brake me. I dont need them and until I finish other upgrades im good here.

Sounds like you need the ratios you cant get. Theres no reason for anything over 4.10 if your gonna be doing what you do. Look into the next best company or one better. Save up alittle more and do that if you have to. From what you are saying that 4.88 and 5.13 stuff is gonna kill your normal driving and fuel economy. If your doing mosty DD'ing why ruin it for something you really dont need. I ride with guys on 4.10's with Rubis that wheel often but dont make a career of it and use there JK's for normal life. One guy even got 37's, he dont complain because he lives with it being kinda sluggish. Its not a race car and I know that you already know that 4.10's are good for you. Dont you??
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Old 04-13-2011, 08:29 AM   #9
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Well I got this chart and post off another forum from a poster named planman. Hope he doesn't mind me posting it here. Still wish I could find and example of MPG's with 33's and 4.88



First, here is the rear wheel horsepower and torque curve information for a JK (a stock tuned JK engine is the bottom blue line):

Notice how steep the horsepower curve is and how incremental increases in rpm make significant increases in horsepower.

Next, assuming a 35" tire measures 34" when mounted to a wheel, set at an appropriate psi, and run on a JK, 70 mph produces the following rpm levels:

Automatic with 4.88s: 2330 rpm
Automatic with 5.13s: 2450 rpm
Automatic with 5.38s: 2569 rpm

For comparison, stock 6 Speed with stock tires: 2533 rpm

The corresponding rear wheel horsepower at 70 mph with a stock tune is:

Automatic with 4.88s: 68 rear wheel horsepower
Automatic with 5.13s: 75 rear wheel horsepower
Automatic with 5.38s: 80 rear wheel horsepower

For comparison, a stock 6 Speed Rubi with stock tires: 80 rear wheel horsepower

68 rear wheel horsepower with an automatic with a stock tune and 4.88 gears is not enough to hold speed on highway grades or windy situations with a heavy, non-aerodynamic JK. Add heavier tires, armor, and gear, and even more horsepower is needed to prevent frequent downshifting.

When the transmission downshifts (flashes the converter), it allows additional slippage of in the transmission to run at a higher rpm than it otherwise would if the overdrive was turned off and the engine was in 3rd gear. The throttle is also increased in combination with the higher rpm when the tranny downshifts.

As a result, automatic transmission downshifting (flashing the converter) has a much more detrimental affect on fuel economy than running 239 more rpm (4.88s vs. 5.38s) with minimal downshifting due to the higher horsepower.

Increased downshifting is also hard on the transmission--resulting in overheating for some JK owners without the addition of a tranny cooler.

The JK engine produces very low horsepower at lower rpms. The automatic transmission overdrive gear is simply a poor match for stock gearing.

Jeep/Chrysler was lazy in having the same differential gearing for automatics and 6 speeds. To match the performance of the 6 speeds, the automatics should have come stock with 4.56s or even 4.88s with stock 32" tires on a Rubi.

This is also why we constantly read threads from guys complaining about the lack of power in the JK engine--nearly all of them have automatics.

The purpose of this post for people to be able to make choices based on factual information rather than anecdotal experience or speculation.

Guys with automatics and non-Rubi's who do not upgrade to Dana 44 front axles are limited to 5.13s and stuck with lower performance, and worse fuel economy.
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Old 04-13-2011, 09:42 AM   #10
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Doesn't quiet answer my size tire issues but thought this was some good info I found in my search from the Jeepforum

-An automatic with 35's would have to gear to 4.56's just to get to the performance of a standard tranny with 3.73's and 35's.
-An automatic with 35's would have to gear to 5.13's just to get to the performance of a standard tranny with 4.10's and 35's.
-An automatic with 35's would have to gear to 5.38's just to get to the performance of a standard tranny with 4.88's and 35's.

-A standard tranny with 35's and 3.21's is a dog...
-An auto tranny with 3.21's does not exist...
-A standard tranny with 35's and 3.73's is a doable set up (not a dog).
-An auto tranny with 35's and 3.73's is a dog...
-A standard tranny with 35's and 4.10's is a decent set up (lower end of stock performance)
-An auto tranny with 35's and 4.10's is a dog...
-A standard tranny with 35's and 4.56's is a good set up (higher end of stock performance)
-An auto tranny with 35's and 4.56's is a doable set up (not a dog)
-A standard tranny with 35's and 4.88's is an ideal set up (lower end of high performance set up)
-An auto tranny with 35's and 4.88's is a doable set up (close to stock)
-A standard tranny with 5.13's should be running 37's (getting to the highest end of high performance)
-An auto tranny with5.13's has just gotten to a stock set up (the lower end of stock)

The 6 speed manual offered in JK's is the NSG370. It's a Mercedes-sourced unit that was also used in 05-06 TJ's.

Gear ratios:
1st - 4.46
2nd - 2.61
3rd - 1.72
4th - 1.25
5th - 1.00
6th - .84
Reverse - 4.06

The 4 speed auto used in the JK's is the 42rle. It was used in 03-06 TJ's as well.

Gear ratios:
1st - 2.84
2nd - 1.57
3rd - 1.00
4th - 0.69
Rev - 2.21
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Old 04-13-2011, 09:49 AM   #11
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Now thats a good set of figures and a great comparison--

In 3 weeks' I'll have a set of YUKON 5.38s in my Rubi and I'll try to post some results!

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Old 04-13-2011, 10:00 AM   #12
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5.38s are not available for the D30. Lowest you can go is 5.13. Need a D44 to run 5.38s.

I have 33s, and plan on 35s down the line. My regear will be 5.13 when the time comes. I am an auto.
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Old 04-13-2011, 11:51 AM   #13
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Well, just got back from the shop. Put in my order and down payment. Getting 4.88 Yukon gears and front Eaton E-lockers. Scheduled to be put in on May 2. Can't wait to try it out.
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Old 04-13-2011, 12:32 PM   #14
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Well, just got back from the shop. Put in my order and down payment. Getting 4.88 Yukon gears and front Eaton E-lockers. Scheduled to be put in on May 2. Can't wait to try it out.
Cool cant wait to hear what you you have to say. Our set up's are similar and I'm still wondering what to do myself. I wat to go with the 44's and 4.10's as I feel that will be good enough for me. I really dont mind the 3.21 that much as it is. I also get over 20 mpg on the highway if I run 6th and cruise steady at 70mph. My read out will say 22+ but its really around 20-21 when I do the math. When I need alittle more power I just go to 5th and deal with it bieing alittle sluggish. The majority of the wheeling I have done hasnt challanged me to say "if I just had better gearing". When in 4wd the power is ample for the most part.
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Old 04-13-2011, 02:10 PM   #15
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Now that I checked with Motive Gear ( all I can get from the store) I can only go from 4.56-5.38 for a Dana 44 in a JK.
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Old 04-13-2011, 02:26 PM   #16
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Congrats Rics1997--looking forward to the review! This has become a really informative thread.

While we're on the topic, I have a related question for anyone who might be interested.

My 4 door 6 speed has the lowly 3.21s and the stock BFG mud tires, and is my daily driver. I actually don't feel particularly underpowered, principally due to the 6 speed. However, like many others, I'm planning on going to 33s (no intentions of ever heading to 35s), and am concerned they'll overwhelm the 3.21s.

I'd like to regear, but "the charts" (like the one I posted above) all seem to suggest 4.10 would really be the ideal set up for 33s, or maybe some non-existent 4.30s or some such thing. But I'm worried 4.56s may be too much.

What's the general thought here? Should I try to find--somewhere and at too high a price--aftermarket 4.10s, or should I just head to 4.56s? Am I going to hate 4.56s on a 6 speed with 33s? Particularly if I have 32s on there for a while between the gear change and the new tires?

There really should be something between 4.10 and 4.56 . . . .
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Old 04-13-2011, 04:50 PM   #17
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I'm in the same boat as you MTH I keep going back and forth on some issues. I might just keep the front 30 get some upgraded shafts, weld up the axle and lock just the rear. The only re gear I can easily get for the front 30 is 4.88 and I dont really care to go there. I also do not have an issue with my 3.21 gears with the 6spd. I wheel this Jeep and I'm not gonna do any ridiclous crawling and have not h ad a problem with the 3.21 yet.
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Old 04-13-2011, 05:16 PM   #18
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^^Exactly.

Insofar as regearing is concerned, it seems hard to win with a 6 speed unless you go all the way to 35" tires. 4.10s are really tough to find aftermarket, and 4.56s look to be mighty deep for 33s.

Anybody out there regeared a six speed and gone to 33s?
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Old 04-13-2011, 06:10 PM   #19
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I dont really see the need to jump to 4.56 like you MTH, overkill for me and will ruin the fuel mileage and i dont really want to go to 35's. Taking alot of notice on my ride home concerning the power and 255/75/17 tires I just dont really feel under powered, that much. Its slow and you need to watch what gear you are in. The majority of my 17 mile ride is between 50-60mph and I cruise in 6th gear.

65mph in 5th gear 2200 rpm in 6th gear 1800rpm
70mph in 5th gear 2400 rpm in 6th gear 2000rpm

I feel my Jeep comes alive around 2000 and develops power at 2200

I typically cruise in 6th between 1500 and 1700rpm.
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Old 04-13-2011, 06:43 PM   #20
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Quote:
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I dont really see the need to jump to 4.56 like you MTH, overkill for me and will ruin the fuel mileage and i dont really want to go to 35's.
That's exactly my problem--I don't see the need to jump to 4.56s either. I only want to run 33s, so if I could readily pick up 4.10s, I'd do that.

I'm stuck in the middle: If I want 33s, my 3.21s might not be enough but 4.56s might be too much. (Recall my four door is quite a bit heavier than your two door). I really think 4.10s would be ideal--but they're really tough to find aftermarket. What's a guy to do?
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Old 04-13-2011, 06:50 PM   #21
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I hear ya MTH. My plan of grabbing a Rubi front 44 got shot down when I found out that getting 4.10 for my rear wasnt really going to easily happen other than finding 2 Rubi diffs.
I think im just gonna get the beefed up shafts, weld it up and run it how it is on the 33's, mabey go to 315/70/17 if I get a deal on some wheels I like and lock just the rear. I can run greens and blues at my "local" ORV park and run the pines no problem like I am so why go extreme with gears and axles. I need to address my drive shaft issues first any ways due to the lift I'm running.

I'm looking forward to Rick getting his Jeep done, the 3 of us are in similar situations.
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Old 04-13-2011, 07:29 PM   #22
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Well at least Yukon makes 4.56 for the D44's. They don't for the D30's though. One of the reasons I didn't get 4.56 was because of that. Both 4.10 and 4.56 are hard to get for the d30's.
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Old 04-13-2011, 08:29 PM   #23
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I don't know what you guys are really pondering about--

If I was in your position, I'd get 4.56, and FAGETABOUTIT-

My Rubi 4.10s will be for sale in a little over 3 weeks and thats with the bearing sets--10,000 miles !!

I don't know about your Dana 30s, it's beyond me !!

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Old 04-13-2011, 08:41 PM   #24
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Quote:
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I don't know what you guys are really pondering about--

If I was in your position, I'd get 4.56, and FAGETABOUTIT-
Wouldn't our six speeds then be turning ~2800 RPMs at 70 mph on true to size 32s? Sounds a little high for a daily driver. Might be hard to fagetaboutit . . . or maybe not . . . I'm definitely torn about this. Hmmm.
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Old 04-13-2011, 08:56 PM   #25
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My Rubi turs 3000 rpm at 70 (4.10) with O/D off and I still average 15mpg and my mileage dash ind showed 16.3 MPG on I80 to/thru Sacramento on I 80


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Wouldn't our six speeds then be turning ~2800 RPMs at 70 mph on true to size 32s? Sounds a little high for a daily driver. Might be hard to fagetaboutit . . . or maybe not . . . I'm definitely torn about this. Hmmm.
With O/D ON it runs 2200 at 70 and I figure the 5.38s will add close to 5oo rpm, it probably will be about 2700 at 70 mph-O/D on !!

2400 to 2600rpm, is supposed to be the "sweet spot" for the 3.8L in cruising/mileage-

Don't forget, it's easy for me, cause mine isn't a DD and I spend little time on the hiwy-

Don't that just piss ya off ??

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Old 04-13-2011, 09:07 PM   #26
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In this case JIMBOX does not see our side of this one. He has a Rubi with 4.10 AUTO that is not his DD as he has stated. Totally different animal than we drive. And as long as you understand these are not race cars the 3.21 with a 6spd is not that bad, I believe we agree on that. I am on pace to put 26000 miles on my JK in my first year owning it. I do not really care about gas as I dont pay for it directly out of pocket but I do not like getting over 2500 rpm in my JK and do drive highway often. I have a second vehicle ( 93 XJ) and my JK is my DD. I do all the extremes with it and its my best friend and companion in my life. No matter where I go I go there in it. I do not want to high a gear that I cant do 80 down the lovely NJ interstates. I also do not want to damage my front 30 with the type of wheeling I do now. The simple answer for me is to beef up the front 30 like stated and just lock the rear and keep with the 33" tires. Or not lock it. Again just a toy. All this will suit me in every aspect of my life and I can run all the trails and parks like I have been and venture into some better stuff without worry.

Keep in mind this thread is about 285/70/17 tires and not 35". No gear upgrade is really necessary and it will only be for "fun"

Like I said cant wait to hear from Rick.
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Old 04-13-2011, 09:27 PM   #27
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i will be putting 285 70 17's on AEV wheels early next week and I have stock 4.10's. I will share my experience as soon as they're on. at this point my ride is DD, but things will change soon....
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Old 04-13-2011, 09:31 PM   #28
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i will be putting 285 70 17's on AEV wheels early next week and I have stock 4.10's. I will share my experience as soon as they're on. at this point my ride is DD, but things will change soon....
Hit the pines yet? Theres a group I ride with that is local to you. From another site but we ride with anyone. Pm me if you dont know what i'm talking about
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Old 04-13-2011, 09:32 PM   #29
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Originally Posted by JIMBOX View Post
My Rubi turs 3000 rpm at 70 (4.10) with O/D off and I still average 15mpg and my mileage dash ind showed 16.3 MPG on I80 to/thru Sacramento on I 80

With O/D ON it runs 2200 at 70 and I figure the 5.38s will add close to 5oo rpm, it probably will be about 2700 at 70 mph-O/D on !!
What size tires are you running? Stock tires and gears should be around 3,000 RPM at 70 with O/D off and 2,100 with O/D on.

I'm running 35s with 5.38s and am at 2,550 at 70 mph with O/D On.
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Old 04-13-2011, 09:47 PM   #30
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Just realized Rick has an Auto, I'm a douche bag and may never post on this forum again. I have more than one reason to not but that just goes to show that what I do for an escape I miss out on the whole point. Sorry to steal your post and good luck with you Jeep

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