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An Engineer's Ultimate Guide To 3.21 VS 3.92 Axle Ratio

Rebelguy2020

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Going from 32" tires to 35's should have netted you a 3.56 ratio. My 33's turned my 3.92's into a 3.79.
My Rebel came with 33” OEM, and came with the 3.92 OEM, now that my tires are worn down 50% my rear end axle ratio magically changed to almost 4.10 I can now tow 1,000 lbs more just by wearing down my tires!
Maybe I’ll machine down the diameter of my rotors to fit 13 inch wheels, then my truck will have 99.2 ratio, then hitch up to the earth, then pull the world to reverse the direction, it will then turn back time to the original post, and delete it! 😳
 

Mountain Whiskey

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Now that is just crazy. Just going to smaller rims won't make a difference unless you reduce tire size too.

Besides, we all know you would need a TRX to pull us backwards anyways.
 

Redfour5

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Which begs the question:
Isn't LSD and traction control kind of redundant anymore?
I can see before traction control existed the huge advantage of LSD, but what's the point of a LSD with traction control? Isn't the 'computer' figuring out which wheel to give power to?
My 2013 5.7 quad cab did NOT have LSD but did have 4WD AND traction control. At the mailboxes where everyone stops and polishes the ice, it would just spin. I'd have to put it in 4WD to get moving. The 2015 crew cab with 4WD and LSD would just shift the power to what ever wheel had traction on take off. You could feel it do it.

So, that's my personal example.
 

theblet

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I would say LSD and BLD would be redundant. Traction Is not the same thing.
 

Scram1500

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Traction control is okay, but nothing like mechanically locking the wheels together. In extreme situations traction control is worthless and could possibly glaze a rotor or cook the pads. My last Ram with an open diff was like a turtle on its back if the road was slightly moist, the Goodyear RSAs which turn to plastic could have played a part as well
 

40Mopar

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I hope this post will help to end the debate with facts and not opinions, and become THE post people refer to those who are having a hard time deciding.

You already know that 3.92 is better for towing, and 3.21 gets better fuel economy, so I will talk about what you might not know

Bottom line up front:

In layman's terms, to conceptualize the difference, imagine

1) 5 out the of 8 gears have the same final drive ratio between 3.21 and 3.92.

2) 3.21 has "an extra" overdrive gear.

3) 3.21 has 2 unique lower gears for towing.

4) 3.92 has 3 unique lower gears for towing.

5) Speed range that 3.21 is better at towing: 31-38 MPH, 48-57 MPH.

6) Speed range that 3.92 is better at towing: 0-30 MPH, 39-47MPH, 58-70 MPH.


Explanation


1) 5 out the of 8 gears have the same final drive ratio between 3.21 and 3.92:

Here's the gear ratio for the 8 speed transmission:
1) 4.71:1 2) 3.14:1 3) 2.10:1 4) 1.67:1 5) 1.29:1 6) 1.00:1 7) 0.84:1 8) 0.67:1 Reverse) 3.30:1

Final drive ratios with 3.21

1st. 15.12, 2nd. 10.10, 3rd. 6.74, 4th. 5.36, 5th. 4.14, 6th. 3.21, 7th. 2.70, 8th. 2.15, R 10.6

Final drive ratios with 3.92

1st. 18.46, 2nd. 12.31, 3rd. 8.23, 4th. 6.55, 5th. 5.06, 6th. 3.92, 7th. 3.29, 8th. 2.62, R 12.94

From the list below, we can see that gears 3-7 in 3.21 matches gears 4-8 in 3.92:

-- NO MATCH -- = 18.46 - 1st - 3.92
3.21 - 1st - 15.12 = -- NO MATCH --
-- NO MATCH -- = 12.31 - 2nd - 3.92
3.21 - 2nd - 10.1 = -- NO MATCH --
-- NO MATCH -- = 8.23 - 3rd - 3.92
3.21 - 3rd - 6.74 = 6.55 - 4th - 3.92
3.21 - 4th - 5.36 = 5.06 - 5th - 3.92
3.21 - 5th - 4.14 = 3.92 - 6th - 3.92
3.21 - 6th - 3.21 = 3.29 - 7th - 3.92
3.21 - 7th - 2.70 = 2.62 - 8th - 3.92
3.21 - 8th - 2.15 = -- NO MATCH --

2) 3.21 has "an extra" overdrive gear:

The 8th gear in 3.92 is the 7th gear in 3.21, thus effectively mean the 8th gear in the 3.21 is an extra gear to the 3.92.

Meaning, when you go test drive the 3.21 you will have to downshift to 7th to get the same acceleration at 3.92's 8th on freeways. That is why some people complain about how "sloppy" the 3.21 is, because the 3.21 has an extra overdrive gear for fuel economy. If you shift 3.21 in 7th gear, you will get the same acceleration as the 3.92 in 8th on the freeway. No, 3.21 isn't sloppy, you're just in a gear that 3.92 does not have.

3) 3.21 has 2 unique lower gears for towing:

As we know from 1), 5 gears have the same final drive ratio.
You "gain" an overdrive gear, but you "lose" one towing gear.
Here's the final drive ratio for the 2 towing gears.
1st. 15.12, 2nd. 10.10,

4) 3.92 has 3 unique lower gears for towing:

Same logic as the last
Final drive for 3 towing gears.
1st. 18.46, 2nd. 12.31, 3rd. 8.23.

5) Speed range where 3.21 is better at towing: 31-38 MPH, 48-57 MPH,
AND
6) Speed range where 3.92 is better at towing: 0-30 MPH, 39-47MPH, 58-70 MPH:

Calculated towing shift point to be 6000 rpm, if I'm off the logic is the same but the speed will vary.

For towing,
From the speed 0-30 MPH, 3.92 has higher final drive ratio over 3.21 (18.46 vs 15.12) until it has to shift to 2nd gear at 30MPH.

From the speed 31-38 MPH, 3.21 has higher final drive ratio over 3.92 (15.12 vs 12.31) until it has to shift to 2nd gear at 38MPH.

From the speed 39-47 MPH, 3.92 has higher final drive ratio over 3.21 (12.31 vs 10.10) until it has to shift to 3rd gear at 47 MPH.

From the speed 48-57 MPH, 3.21 has higher final drive ratio over 3.92 (10.10 vs 8.23) until it has to shift to 3rd gear at 57 MPH.

From the speed 58-70 MPH, 3.92 has higher final drive ratio over 3.21 (8.23 vs 6.74) until it has to shift to 4th gear at 70 MPH.

The key takeaway here is that towing heavier trailers uphill with 3.21 might never reach the desired speed within the 58-70 MPH range (typical highway towing speed) because 3.21 jumps from 10.10 to 6.74 without the 8.23 final drive ratio found in 3.92 that really help maintaining highway towing speed at max load.

Do you value the "extra" overdrive gear for fuel economy? or do you value the extra towing capability that you tell yourself you might one day need? That's up to you.
Great post and most importantly it is what is " just the facts" ! Very helpful information thank you for taking the time to write it up it helped me.
 

Robbyreneeward

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Meanwhile, back at the ranch……

So I’m ordering a 22 Laramie, going to level the front 2” and switch stock tires for 295/65/20 (35.1x11.6), and adding A/T tires that weigh 30# more than stock. I’m assuming I’ll need to order 3.92 gears to essentially be set back to 3.56 or so, correct? I’m also guessing a loss of 2-3 MPG? This is with a 3.0 btw. Had a 20 diesel wrangler and averaged 27-28 until I lifted and added 37s. Mileage dropped to 21-22 at that point. Thoughts please?


Sent from my iPhone using Tapatalk
 

ChrisID

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I'll add my 2 cents to this debate. I own both. A '20 Laramie 3.9, and a '21 Limited 3.2, located in different states. They are very close in drivability unloaded. However, if I just put 1-2 dirtbikes in the 3.2, it shifts constantly on the highway, sometimes down to 4-5th, while the 3.9 doesn't go lower than 6th. I don't really care, just an observation.
Towing 5-6K, the 3.2 shifts every second it seems (in Appalachians), and it goes to 3rd sometimes to maintain 50-60 rural roads.
The 3.2 still does the job well, but mpg is pretty bad at 8-10 towing heavy (lower than my past 4-5 trucks). Even just dirtbikes in the back it's 12-14 (this is odd to me, as all prior trucks did better with just bikes). The 3.9 appears to be a little better by 1-2mpg. But I don't pay a lot of attention to mpg. Again, just an observation.
The only downfall I see to the 3.9 is at 80mph is tachs up higher (2400?).
After owning both for over a year, and both over 10K miles, I prefer the 3.9's by a small margin. If I had to put number on it, probably 20% better. If I were ordering another one, it would be 3.9.
However, if there were two trucks side by side to buy and the 3.2 was $4-5K lower, I'd take it. It's not that big of a deal.
Towing heavy all the time? 3.9's for sure.
 

theblet

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I'll add my 2 cents to this debate. I own both. A '20 Laramie 3.9, and a '21 Limited 3.2, located in different states. They are very close in drivability unloaded. However, if I just put 1-2 dirtbikes in the 3.2, it shifts constantly on the highway, sometimes down to 4-5th, while the 3.9 doesn't go lower than 6th. I don't really care, just an observation.
Towing 5-6K, the 3.2 shifts every second it seems (in Appalachians), and it goes to 3rd sometimes to maintain 50-60 rural roads.
The 3.2 still does the job well, but mpg is pretty bad at 8-10 towing heavy (lower than my past 4-5 trucks). Even just dirtbikes in the back it's 12-14 (this is odd to me, as all prior trucks did better with just bikes). The 3.9 appears to be a little better by 1-2mpg. But I don't pay a lot of attention to mpg. Again, just an observation.
The only downfall I see to the 3.9 is at 80mph is tachs up higher (2400?).
After owning both for over a year, and both over 10K miles, I prefer the 3.9's by a small margin. If I had to put number on it, probably 20% better. If I were ordering another one, it would be 3.9.
However, if there were two trucks side by side to buy and the 3.2 was $4-5K lower, I'd take it. It's not that big of a deal.
Towing heavy all the time? 3.9's for sure.
thats my point. 3.21 for occasional towing and better mpg on the highway. 3.92 for lots of towing/big tires. It's a preference. No right or wrong answer.

And this thread lives on!
 

Idahoktm

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I'll add my 2 cents to this debate. I own both. A '20 Laramie 3.9, and a '21 Limited 3.2, located in different states. They are very close in drivability unloaded. However, if I just put 1-2 dirtbikes in the 3.2, it shifts constantly on the highway, sometimes down to 4-5th, while the 3.9 doesn't go lower than 6th. I don't really care, just an observation.
Towing 5-6K, the 3.2 shifts every second it seems (in Appalachians), and it goes to 3rd sometimes to maintain 50-60 rural roads.
The 3.2 still does the job well, but mpg is pretty bad at 8-10 towing heavy (lower than my past 4-5 trucks). Even just dirtbikes in the back it's 12-14 (this is odd to me, as all prior trucks did better with just bikes). The 3.9 appears to be a little better by 1-2mpg. But I don't pay a lot of attention to mpg. Again, just an observation.
The only downfall I see to the 3.9 is at 80mph is tachs up higher (2400?).
After owning both for over a year, and both over 10K miles, I prefer the 3.9's by a small margin. If I had to put number on it, probably 20% better. If I were ordering another one, it would be 3.9.
However, if there were two trucks side by side to buy and the 3.2 was $4-5K lower, I'd take it. It's not that big of a deal.
Towing heavy all the time? 3.9's for sure.
It's 1900 rpm's at 75 mph and if I remember correctly, it's 2200 rpm's at 80 mph.
 

Scram1500

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It's 1900 rpm's at 75 mph and if I remember correctly, it's 2200 rpm's at 80 mph.
The 305/55 r20's you have has changed your gear ratio from 3.92 to 3.70ish something so your rpm/speed will be different than stock. From what I recall with OEM 32inch tires 1900 rpm is 70-72 mph
 

IvoryHemi

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The 305/55 r20's you have has changed your gear ratio from 3.92 to 3.70ish something so your rpm/speed will be different than stock. From what I recall with OEM 32inch tires 1900 rpm is 70-72 mph

The rpm’s will look the same on the tach, won’t be noticeable.

Going from 31.9” to 33.2” tire would result in a <100 rpm drop at 70mph
 

Ramjack

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The rpm’s will look the same on the tach, won’t be noticeable.

Going from 31.9” to 33.2” tire would result in a <100 rpm drop at 70mph
That's more true than you may realize because the speedo will read artificially high as well since it is based on rotations and not actual ground speed.
 

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