Axle gearing, ratios, 35s, and model differences - JKowners.com : Jeep Wrangler JK Forum
 
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post #1 of 15 Old 04-15-2009, 09:43 AM Thread Starter
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Axle gearing, ratios, 35s, and model differences

I was a bit bored and mulling over how everyone and their brother expresses that new gears are basically required if you go to 35s, even in Rubis. I have a Rubi with a 6-speed and have been entertaining the idea of 35s – and of course, wondering if I really need new gearing as part of the package. So I put together a hypothetical comparison!

From an academic standpoint, if you want to maintain stock performance when going from 32s w/4.10s to 35s…people will say you need 4.88s to regain stock performance, probably because the added rotating mass and unsprung weight, but from an academic standpoint the equivalent new ratio is 4.48.

Here’s the hypothetical situation: Your buddy has a Sahara with 3.73s and you have a Rubicon. Both are manuals, and you both are going to 35s. You are keeping the 4.10s, he will re-gear. Both have the factory transfer cases at 2.72:1 and 4.0:1 ratios, respectively.

Adding 35s to a 4.10 axle brings the equivalent gear ratio to 3.75, so you’re pretty much even with your stock Sahara buddy with his 32s. The 35s go on.

Sahara wants to maintain stock overall ratios, and the calculator tells him that the equivalent axle ratio with 35s will be 4.08. So he buys a set of Rubi take-off 4.10s and installs them along with the 35s.

Although not technically related to tire size…your Rubi’s overall crawl ratio has been reduced from 73:1 to 67:1. Still a very low ratio suitable for heavy wheeling work. Sahara’s crawl ration is 45:1, roughly the same as stock.

In 4-lo and in first gear, at 3000 RPM your Rubi’s speed has increased from 3.9 MPH with stock tires to 4.27 MPH with 35s, less than ½ MPH difference. Sahara’s speed under the same conditions is 6.28 MPH.

At 65 mph in 6th gear, Rubi’s RPMs have dropped from 2350 to 2150. Stock with 32s and 3.73s, Sahara’s RPMs under the same conditions is 2138 (and the same now due to the gear change). This puts Rubi’s highway revs well within the factory spec range.

Conclusion? Putting 35s on a stock geared Rubi only very slightly changes the the 4-lo ratios and speeds, where it still has a distinct advantage over the Sahara at least on paper. Outside of 4-lo, it behaves just like a stock Sahara with 32s and 3.73s which is obviously well within the vehicle design limits and specifications. Your ‘butt-dyno’ probably will tell a slightly different story with the added unsprung weight and rotating mass, but I’d hardly consider a gear upgrade a necessity for a Rubi to move to 35s.

Now, if you have an AUTO….let’s just say I feel your collective pain. Plug in those ratios and the game changes big time. In that scenario I can totally see why you could consider gears a necessity. A stock Rubi crawl goes from 73:1 to 46.5:1 when you have an auto…for a Sahara you’re now at 28.8:1 from 45:1. Yikes.

If you’ve read this far, you’re as bored as I.
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post #2 of 15 Old 04-15-2009, 09:50 AM
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good post!

I have the 2 door 6 speed rubi on 35's w/o re-gearing and its noticable but no way mandatory to regear. If I had the extra loot laying around Id like to, and probably will someday, but not soon.

Also good to see the crawl ratio effects, I was wondering about how those would change on paper.


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post #3 of 15 Old 04-15-2009, 10:01 AM
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Two things to consider.

-Unsprung weight is largely mis-understood. There is a reason 95% of the vehicle is built above the springs. If it were only a matter of comfort then only the seats would have springs on them. The ratio changes from one vehicle to the next varies but a good "rule of thumb" is that 1lb of unsprung weight is equal to 10lbs of sprung weight.

What does this mean? I can give you the math for my TJ since I have not weighed the stuff for the JK. Going from the stock 30s to 33s added 17lbs per wheel of unsprung weight. Which roughly equals 650lbs of effective weight added to the Jeep that it must now accelerate, stop and turn.

Add to all this the extra rolling resistance of sticky, fatter rubber and the wind resistance increase of the now larger vehicle and you will see where the need starts to come from.


Now, the second thing. The Auto has an INSANELY deep overdrive compared to the stick. Run this number. A Rubi, 4:10 6 speed with 35s rolling down the highway is 40 rpm(yes, 40) less than an Auto with 5:13s and 35s.



....just a side note on your comparison. A Rubi had a heavier drivetrain and more gear friction to fight through than the Sahara. So that will make some difference too. I am always amazed at the difference when upgrading to bigger axles.

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post #4 of 15 Old 04-15-2009, 10:05 AM
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This is a great read... I've got Rubi/35's/auto and am just trying to throw some cash together to get gears. I can live without, but it isnt the best driving experience. Plus, the new crawl ratio def needs to improve.
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post #5 of 15 Old 04-15-2009, 10:06 AM
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Quote:
Originally Posted by X-Defector View Post
A stock Rubi crawl goes from 73:1 to 46.5:1 when you have an auto…for a Sahara you’re now at 28.8:1 from 45:1. Yikes.
Your forgetting the doubling factor you get from the torque converter, and you can let TC slip in an auto which you can't really do a manual. Working out real world crawl ratios for autos is a little more difficult than it would first appear.
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post #6 of 15 Old 04-15-2009, 10:12 AM
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Around town I did not thing going from 4:10 to 5:13 was that big of a deal. (Auto, 35s, Unlimited).

It actually feels sluggish on the highway....because it is in OD now instead of third.

Now, climing the mountains from Denver to Summit County is a HUGE HUGE difference.

I could hold the 65 mph speed limit before, but the Jeep would run as high as 5K in 2nd gear to pull the hills. Now it can pull them at 3k in 3rd and use OD from time to time.

Overall my mileage has gone up too. (all corrected with AEV Procal)

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post #7 of 15 Old 04-15-2009, 10:31 AM Thread Starter
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All great points. As I mentioned, the numbers I created are basically an exercise in academia, but theory and practice do split somewhat in the real world - particularly when you talk about manual vs auto. And indeed the auto has a super deep overdrive - .69 vs the .84 of the manual.
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post #8 of 15 Old 04-15-2009, 11:52 AM Thread Starter
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Quote:
Originally Posted by Xtremjeepn View Post
Now, climing the mountains from Denver to Summit County is a HUGE HUGE difference.
I haven't yet had the pleasure (torture?) of running this pass with my JK yet. I'll be doing it a lot this summer though, so I hope its not a miserable experience.

Before the JK, I had an '07 Xterra 6-speed (hence the user name). That thing had no shortage of power and made easy work of stuff like Floyd hill. I have a feeling it won't be so easy in the Jeep...
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post #9 of 15 Old 04-15-2009, 01:03 PM
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Nope, I drove my JK cross country with the BB and 33s and I can offer one bit of advice. Draft the big guys alot.

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post #10 of 15 Old 04-15-2009, 08:54 PM
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You need 5.13s for 35s with an automatic. Suddenly you have power to pass without turning off the overdrive. It puts the engine in the sweet spot. That being said, I just installed 37s and I'm back turning off the overdrive.

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post #11 of 15 Old 04-16-2009, 10:12 AM
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i was planning on stuffing 38s or 40s on my 07 rubi will 5.13s be alright with this combo but i was thinking if i go that big i would need to put heavier axles correct d60 or something im a newb so i figure this would be a good thread to ask

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post #12 of 15 Old 04-16-2009, 10:31 AM
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Quote:
Originally Posted by rage344 View Post
i was planning on stuffing 38s or 40s on my 07 rubi will 5.13s be alright with this combo but i was thinking if i go that big i would need to put heavier axles correct d60 or something im a newb so i figure this would be a good thread to ask
D60's would obviously be the better choice, and wider ones too. I'm running 40's on D44's with Superior shafts, front sleeves, gussets, etc, and will see how they do. With an easy right foot and airing down to single digits they may survive, we'll see.

I have 4.88's right now, and they are okay, but 5.38's would be a better choice.

You'll also need to put a sawzall and grinder to work to trim the body for 40's.
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post #13 of 15 Old 04-16-2009, 02:42 PM
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Quote:
Originally Posted by PhilD View Post
D60's would obviously be the better choice, and wider ones too. I'm running 40's on D44's with Superior shafts, front sleeves, gussets, etc, and will see how they do. With an easy right foot and airing down to single digits they may survive, we'll see.

I have 4.88's right now, and they are okay, but 5.38's would be a better choice.

You'll also need to put a sawzall and grinder to work to trim the body for 40's.

cool thanks for the input i love this site y'all are great. Phil you goin to the jk rally next weekend?

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post #14 of 15 Old 04-17-2009, 05:17 AM
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Good post x factor and good follow on points guys.

When I went to 35's it felt like I lost a lot of power. I guess I'm just used to the torque curve of a diesel! It's difficult to use OD for anything but high speed freeway runs now.
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post #15 of 15 Old 04-19-2009, 05:37 AM
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So I have a X with a 6 speed and the standard 3.23 gearing (but I have the upgraded wheels so I am running ~32"). By my calculations if I step up to 35" and want to get roughly stock 4.10 gearing (like the towing package). I would need ~4.48 gears.

Is this correct math?

(new wheel/stock wheel)*OEM gear = new gear

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