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

[Boston]

Not goin to roam pages of text etc, but what about 2wd v 4wd vehicles?

4wd after all has a transfer box with its gear ratios. No transfer box on 2wd I assume.

I definitely get this discussion as Jeep owners constantly compare crawl ratios.

 

[Vasa1]

Hi, here in Canada. I might be buying a 19 bighorn crew and it has the 3.92 with axle lock. I travel the hwy every weekend to the cottage. I plan on installing a 2” lift with 285/60-20 ko2 on it. Just by going to this taller tire, would that change the gearing?

 

[parnine]

Hi, here in Canada. I might be buying a 19 bighorn crew and it has the 3.92 with axle lock. I travel the hwy every weekend to the cottage. I plan on installing a 2” lift with 285/60-20 ko2 on it. Just by going to this taller tire, would that change the gearing?

Yes, would put to 3.74 aprox - stock tires are in the 32" range, and yours are in the 33.5" range, this will change the 'effective gearing' - check out https://tiresize.com/gear-ratio-calculator/

 

[Vasa1]

Yes, would put to 3.74 aprox - stock tires are in the 32" range, and yours are in the 33.5" range, this will change the 'effective gearing' - check out https://tiresize.com/gear-ratio-calculator/

But it should be ok for hwy driving?

 

[parnine]

But it should be ok for hwy driving?

what is 'ok' for you? depending upon your point of view it would be 'better', or 'worse', than stock for hwy driving

 

[zchamber]

Vasa1: Perhaps you want it to turn less rpm on hwy speed? you will be about 2k at 75mph, a lot better than ~2200 before with the OEM and 3.92?

 

[Wrecktal]

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.

Do the numbers change at all when it's and EcoDiesel and not the 5.7? The RPMs will be massively lower.

 

[Boston]

Not goin to roam pages of text etc, but what about 2wd v 4wd vehicles?

4wd after all has a transfer box with its gear ratios. No transfer box on 2wd I assume.

I definitely get this discussion as Jeep owners constantly compare crawl ratios.

Well, after further review, the transfer case high range ratio is 1.0-1. So there is no difference between rwd v aWD gear ratios.

 

[MF2020]

Well, after further review, the transfer case high range ratio is 1.0-1. So there is no difference between rwd v aWD gear ratios.

Do the numbers change at all when it's and EcoDiesel and not the 5.7? The RPMs will be massively lower.

Just came across this thread. Excellent job by the OP! I have had both the 3.21 and the 3.92 and all I can say is that I absolutely love how lively the 3.92s feel with or without a trailer. the 3.21s lugged the trailer around pretty good but the 3.92s whip it it around effortlessly especially upon take off and at high speed. Yup, this vehicle sure does use more gas. About 2 miles per gallon more on average but the driving satisfaction for me has been worth the trade off. I certainly don't fault anyone for making a decision to achieve better fuel economy especially if they do not haul a trailer. So there is my very technical, seat of the pants opinion. LOL. Thanks again OP for such a thoughtful and thorough post on this topic.

 

[Boston]

No change in the gear box ratios according to this publication by FCA between the Ecodiesel and Hemi.

 

[CaptainCJ35]

Just came across this thread... awesome write up and data.

 

[BigD]

Won't the 3.92 be faster off the line than the 3.21 without any trailer attached ? I got the 3.92 so I can get in front of someone say if the right lane is ending or something

 

[MF2020]

Won't the 3.92 be faster off the line than the 3.21 without any trailer attached ? I got the 3.92 so I can get in front of someone say if the right lane is ending or something

Well officially I do not know the technical answer to this but I can tell you that I have owned both the 3.21 and the 3.92 and the 3.92 feels substantially quicker off the line! The truck just fells lighter all around because it moves from a dead stop so much easier. Yup, it uses gas... but I don't really care because the performance to me is worth the trade-off! Also, I have slightly bigger tires than stock with a hybrid tread so the 3.92s help.

 

[Tjf2000]

Here are actual caclulations of a 3:92 vs 3:21. 2016 ram has 139k w/3:92 16.28 mpg 2014 ram had 69k 16.56 mpg.

 

[STR]

OP, good write-up

I went from a 4th Gen 3.92 to a 3.31, and now have a 5th Gen 3.92. The only observation I made was that the 3.92 accelerates quicker an feels more responsive. The 3.31 had better gas mileage... and I’m sure it would have a higher top speed, but I ever reached it.

 

[bluecrewnuke]

Who builds these axles? Does the 3.21 have a proven track record?

 

[WXman]

Who builds these axles? Does the 3.21 have a proven track record?

I'm pretty sure ZF makes the 9.25" axles for these trucks now. Used to be AAM up until a few years ago or so.

 

[th365thli]

Just adding my data point and experiences. I have a 2019 Big Horn with the 3.21. I added LT 275/65/20 Falken Wildpeaks and Fuel vapor wheels. That tire and rim combo is heavy. We're talking 66 lbs per tire and 46 lbs per wheel, so 112 lbs on each corner. The truck drives just fine. The 5.7 has plenty of power. Yeah, it's not gonna be as quick, but really, it still drives great. The shifting is not quite as smooth when accelerating, but highway driving it still shifts seamlessly. It holds 8 great, and goes down to 7 on inclines, but it does it so seamlessly I don't even notice.

I tow a 3500lb bass boat as well, and have no issues towing. You'll definitely feel it back there when accelerating from a stop for sure. But the acceleration is more than acceptable. And on the highway it pulls it beautifully. On tow/haul mode, it keeps at around 7 and goes down to 6 on inclines. I drive 70-75 mph while towing. Same seamelss shifting.

I'm not saying 3.91 won't be faster, or won't pull it better. In fact, I was looking for used 3.91 but couldn't find them. But the 3.21 is more than capable running heavy tires and pulling a light/moderate load. Will a 3.91 pull better? Of course, but the 3.21 pushing a 5.7 is more than capable of running larger tires and towing. If you run a lighter set of 35s on some lighter weight wheels you'll be just fine.

This is all bone stock without any tune. I may get a pedal commander or tune down the line, but honestly I just don't see any need for it. I get 15 mpg mixed driving. I get 11 mpg while towing.

 

[JustMe]

Don't want to upset the flow of things, and no, I have not read posts on this thread in awhile, however I do have a "crazy" thought that won't leave my head .
It does involve gearing, but the 3.55 kind . In recent RAM site B.& P. of Big Horns, RAM has decided B.H.'s no longer get to have the Center Console ! ?? . What the ? SO, I've decided (unless things change AGAIN) for the 2021 MY DT's, that I'm now looking at a Quad REBEL w/V6 . BUT the Rebel only has the 3.92's and I'd rather have the 3.55 . Not possible from the factory but doable at an authorized RAM Dealership ? Please no WHY Questions . . . they will probably get "out of hand" quickly and I don't need the distraction . Just wanting a bit more efficiency and willing to take the slight hit in torque & towing . I'm not against even going to 3.21, but every time I B.& P. the Big Horn I select the 3.55 (V6) so to have a bit more (minus the small hit in mpg's ?) . The "dudes" at TFL-truck thing it would be way expensive and would void any warrantee !?. Thoughts ? I'll be turning the truck into my Overland Camping Rig Full time, as my "Daily Driver" (only vehicle) . So, IT will have about 300 lbs. added Bed Weight (full time) -- Cap, slide out Fridge + camping/fishing gear . . . ! I do eventually plan on removing the rear seats, reducing some of that weight, where the fridge may eventually be relocated (centering the weight some .
Back to the Question -- switching Rebel from 3.92 to 3.55 ? good bad or indifferent ? Warrantee issue or not ? Possible Cost involved ? thanks . . . stay healthy, be safe ALL .

 

[JJRamTX]

Won't the 3.92 be faster off the line than the 3.21 without any trailer attached ? I got the 3.92 so I can get in front of someone say if the right lane is ending or something

The 3.92 is faster off the line and most likely to 60 and 100MPH but I haven't seen a drag race comparison. With the eTorque and 3.92 you have a even slightly quicker off the line also.