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

[theblet]

The 4wd auto function is in the transfer case and uses clutches to send power to front wheels when rear wheel slip is detected.

Correct. And applies the brakes to the slipping wheel to cause the opposite wheels to pull for traction

 

[oldsouth]

Just read my manual more thoroughly. My 2020 has the TCS system and I have the limited slip axle option. How do these two systems work together in the rear axle? Seems like the TCS would not activate in the rear end with the limited slip. I have only driven once in ice and snow (Louisiana) and with the 4 wheel drive in auto, the truck didn't spin a lick in ice so slippery you could hardly walk on.

 

[theblet]

Traction control doesn’t operate the differentials. It kills power to the engine for a bit and applies brakes if you lose traction.

I live in Louisiana too. Hope you’re not in hurricane ida’s path.

 

[1awesomelaw]

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.

OK, so from an mechanical engineering stand point, which would you prefer gear wise 3.21 or 3.92 if you never will tow with your truck - My son asked ? I purchased my truck with 3.92.

 

[silver billet]

OK, so from an mechanical engineering stand point, which would you prefer gear wise 3.21 or 3.92 if you never will tow with your truck - My son asked ? I purchased my truck with 3.92.

Is your son going to rock crawl or build out his truck by doing a cam, super charger... in other words, does he really care about 1/10th of a second quicker? If not, 3.21 is the best ratio for 99%(*) of people out there.

(*) number pulled from my butt, not statistically measured nor tested on children under the age of 12.

 

[Mountain Whiskey]

OK, so from an mechanical engineering stand point, which would you prefer gear wise 3.21 or 3.92 if you never will tow with your truck - My son asked ? I purchased my truck with 3.92.

First, never look at it from an engineers view. It won't make sense and will likely just be wrong.

Stick with the 3:92 as you purchased. It is far superior in every way. It will make his truck more powerful and better. Don't let him be a wussy crying that in a lab situation you could theoretical get a quarter fraction of a percent of milage better by sacrificing your own manhood.

 

[oldsouth]

That's what I would like to see... 0-top end with the same engine, identical trucks, 3.23 vs 3.92. Then do a fuel economy run, preferably longer than 200 miles.

I'm hopefully not in Ida's path... In North Louisiana. How about you?

 

[HandyCruiser]

I was in the middle of doing my own engineering write up for this post. Then I realized the OP did that much better than mine in the first post. And it's excellent. While we have the 3.92 rear axle ratio, I actually value the "extra" overdrive gear for fuel economy. I don't need the extra towing capability that the 3.92 rear end provides. If the truck had faster throttle response and more aggressive but moderate exhaust, it would be fine for our use.

 

[Rlaf75]

I was in the middle of doing my own engineering write up for this post. Then I realized the OP did that much better than mine in the first post. And it's excellent. While we have the 3.92 rear axle ratio, I actually value the "extra" overdrive gear for fuel economy. I don't need the extra towing capability that the 3.92 rear end provides. If the truck had faster throttle response and more aggressive but moderate exhaust, it would be fine for our use.

You can buy an aftermarket throttle controller to get the throttle to respond quicker. There's several different ones available but they all do the same thing. Pedal Commander is probably the most popular but there are much less expensive brands available. I'm actually researching g this now for my truck

 

[Rlaf75]

Just read my manual more thoroughly. My 2020 has the TCS system and I have the limited slip axle option. How do these two systems work together in the rear axle? Seems like the TCS would not activate in the rear end with the limited slip. I have only driven once in ice and snow (Louisiana) and with the 4 wheel drive in auto, the truck didn't spin a lick in ice so slippery you could hardly walk on.

Traction control is basically the ABS controlling the wheel speed. When it senses excessive wheel speed at one wheel it will apply brake pressure to that wheel slowing it down therefore reducing its spin to gain traction

 

[theblet]

3.21 is fine for towing I sure you. Plus better mpg

 

[silver billet]

First, never look at it from an engineers view. It won't make sense and will likely just be wrong.

Stick with the 3:92 as you purchased. It is far superior in every way. It will make his truck more powerful and better. Don't let him be a wussy crying that in a lab situation you could theoretical get a quarter fraction of a percent of milage better by sacrificing your own manhood.

Gee Whiskey, you sure are worried about your uh, "manhood" Compensating perhaps?

A tip for you, grab the 3.21 to save hundreds each year on gas and extend the life of your hemi.... and for your manhood problem, just roll with the mirrors flipped out.

 

[HandyCruiser]

You can buy an aftermarket throttle controller to get the throttle to respond quicker. There's several different ones available but they all do the same thing. Pedal Commander is probably the most popular but there are much less expensive brands available. I'm actually researching g this now for my truck

Yep. On my Charger R/T Daytona, I have a tune that increases throttle response a lot. Even though it just has a 5.7, it feels more powerful than the 392 that was in our Scat Pack due to the quicker throttle. I figure the Pedal Commander is the way to go with this new Ram since it doesn't involve tuning the PCM and can be easily removed before service at the dealership not to impact the warranty on the truck. But I'm in no hurry. We are possibly taking another road trip out west this fall. I may get a Pedal Commander before we go.

 

[lkjk]

OK, so from an mechanical engineering stand point, which would you prefer gear wise 3.21 or 3.92 if you never will tow with your truck - My son asked ? I purchased my truck with 3.92.

Gonna say it depends where you live as well. In the mtns, i want 3.92s. I also plan on running heavier and larger tires than stock in a few weeks, again wanting the 3.92s.

For the "save 100's a year" comment. At $3.50 a gallon and 12k miles per year, comparing 16 vs 18 mpg is a difference of $292 a year in gas. If gas prices go down, that shrinks as well. Personally, if your whole argument is for mpg, which is because you want to save $$, that's not enough to matter. no one's life is changing over an extra $300/year.

 

[Idahoktm]

Gonna say it depends where you live as well. In the mtns, i want 3.92s. I also plan on running heavier and larger tires than stock in a few weeks, again wanting the 3.92s.

For the "save 100's a year" comment. At $3.50 a gallon and 12k miles per year, comparing 16 vs 18 mpg is a difference of $292 a year in gas. If gas prices go down, that shrinks as well. Personally, if your whole argument is for mpg, which is because you want to save $$, that's not enough to matter. no one's life is changing over an extra $300/year.

I ordered 3.92 gears for the same reason, plus I wanted snappier acceleration. 95% of my driving is around town, so I'm thinking the difference in MPG isn't measurable.

 

[silver billet]

Gonna say it depends where you live as well. In the mtns, i want 3.92s. I also plan on running heavier and larger tires than stock in a few weeks, again wanting the 3.92s.

For the "save 100's a year" comment. At $3.50 a gallon and 12k miles per year, comparing 16 vs 18 mpg is a difference of $292 a year in gas. If gas prices go down, that shrinks as well. Personally, if your whole argument is for mpg, which is because you want to save $$, that's not enough to matter. no one's life is changing over an extra $300/year.

It's all minor (where did anyone talk about "life changing"? Lets not make a strawman here); you say the gas difference is minor, I say the acceleration difference is minor. How often do you truly go WOT? As long as you're not doing that, the 3.21 will accelerate and get you up to speed just as good as the 3.92. You guys in a hurry to get to the next stop light?

My truck lives in 8th gear, I cannot imagine driving around in 7th (which is basically what you're truck is doing compared to mine) just to say "but that fraction of a second!!" Every minute I spend in 8th (and literally 95% of my truck spends it's life in 8th) just reinforces that I made the right decision (for me, but for most people as well).

I'll take the savings thanks, small as they may seem to you. I bet it pays for my 89 octane (vs 87 octane) fuel.

 

[nc_beagle]

I

It's all minor (where did anyone talk about "life changing"? Lets not make a strawman here); you say the gas difference is minor, I say the acceleration difference is minor. How often do you truly go WOT? As long as you're not doing that, the 3.21 will accelerate and get you up to speed just as good as the 3.92. You guys in a hurry to get to the next stop light?

My truck lives in 8th gear, I cannot imagine driving around in 7th (which is basically what you're truck is doing compared to mine) just to say "but that fraction of a second!!" Every minute I spend in 8th (and literally 95% of my truck spends it's life in 8th) just reinforces that I made the right decision (for me, but for most people as well).

I'll take the savings thanks, small as they may seem to you. I bet it pays for my 89 octane (vs 87 octane) fuel.

I have no numbers to support this but I imagine in the mountains where I live you would be in 7th a lot whole lot while I am in 8th? On uphills of course. We all get 99mpg on the downhills, LOL.

 

[lkjk]

It's all minor (where did anyone talk about "life changing"? Lets not make a strawman here); you say the gas difference is minor, I say the acceleration difference is minor. How often do you truly go WOT? As long as you're not doing that, the 3.21 will accelerate and get you up to speed just as good as the 3.92. You guys in a hurry to get to the next stop light?

My truck lives in 8th gear, I cannot imagine driving around in 7th (which is basically what you're truck is doing compared to mine) just to say "but that fraction of a second!!" Every minute I spend in 8th (and literally 95% of my truck spends it's life in 8th) just reinforces that I made the right decision (for me, but for most people as well).

I'll take the savings thanks, small as they may seem to you. I bet it pays for my 89 octane (vs 87 octane) fuel.

The only benefit of 3.21s vs 3.92s is mpg, at the expense of performance. The reason people care about MPG is because of the associated expense, ignoring the hassle factor of stopping for fuel.

My point, and the reality, is that there's not a material difference in fuel expense between the two gear ratios.

The other side of the argument is resale. This is speculation, but I would think you would have faster depreciation on a 3.21 vs a 3.92, similar to a v6 vs v8, simply because of the decreased towing capacity.

Do what works for you, but if gas were free, no one would be buying the 3.21s.

 

[KWKSLVR]

It's all minor (where did anyone talk about "life changing"? Lets not make a strawman here); you say the gas difference is minor, I say the acceleration difference is minor. How often do you truly go WOT? As long as you're not doing that, the 3.21 will accelerate and get you up to speed just as good as the 3.92. You guys in a hurry to get to the next stop light?

My truck lives in 8th gear, I cannot imagine driving around in 7th (which is basically what you're truck is doing compared to mine) just to say "but that fraction of a second!!" Every minute I spend in 8th (and literally 95% of my truck spends it's life in 8th) just reinforces that I made the right decision (for me, but for most people as well).

I'll take the savings thanks, small as they may seem to you. I bet it pays for my 89 octane (vs 87 octane) fuel.

I agree 100%. I've never understood the average truck driver that drives like they're in a sports car. You're not. It's not fast. Even a TRX is not all that blazing fast (please bear in mind I come from a background where a respectable quarter mile is high 11's).

Maybe I'm just not in a hurry anymore and I burned it all out of my system when I was a dumb teenager/20 something. I leave on time and get where I'm going early and routinely catch up to pedal to the floor trucks a few red lights later anyway.

I also routinely coax more mpg out of my vehicles than the EPA estimates. It matters to me, especially on long road trips.

 

[BowDown]

The only benefit of 3.21s vs 3.92s is mpg, at the expense of performance. The reason people care about MPG is because of the associated expense, ignoring the hassle factor of stopping for fuel.

My point, and the reality, is that there's not a material difference in fuel expense between the two gear ratios.

The other side of the argument is resale. This is speculation, but I would think you would have faster depreciation on a 3.21 vs a 3.92, similar to a v6 vs v8, simply because of the decreased towing capacity.

Do what works for you, but if gas were free, no one would be buying the 3.21s.

Lots of assumptions here, 1st is performance. How much performance difference do you think there is 3:21 vs 3:92? A couple 100th's on a good day, that's not even a car length yet the trade off in mpg is, percent wise, greater.
Secondly, resale, that's no difference than having 4x4, sunroof, Limited vs Laramie ect, its options and what someone is looking for when they buy. I didn't want a 4x4 3:92 truck however many do however, many also do not so the resale deprecation isn't relevant because that's about what person A wants vs person B.

Personally, I had a hard time selling my last truck with 3:92 because of the fuel mileage, to buyers walked because of that, the guy that bought it, really didn't care about mileage because of how good of shape the truck was in.
If gas were free, I wouldn't be buying a 3:92 either but gas isn't free so that statement is ridiculous

As for performance, I would like to see what the real difference is, (although I know that a 5500lb 400 hp truck equipped with a 3:92 ad a 3:21 isnt going to be much difference in performance) anyone in the Plano/Frisco area with a 3:92 truck, let's make a run and see the difference.
Anyone thinking their 3:92 truck is fast/quick needs a drug test