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

[IvoryHemi]

so, is the Rebel "effectively" a 3.79, or is the regular 1500 a 4.05?

It’s the former.

 

[Cliffrunner88]

Yeah I doubt they’d make a different gear ratio to adjust for a slightly different tire size to be “accurate”

 

[HSKR R/T]

It’s the former.

Depends on how you want to look at it really. You could put on larger tires that "effectively" give a 3.92 geared Ram a 3.21 ratio, but you aren't getting same "benefit" for fuel mileage as a factory 3.21. so the whole "effective" ratio BS, isn't even worth talking about.

 

[IvoryHemi]

Depends on how you want to look at it really. You could put on larger tires that "effectively" give a 3.92 geared Ram a 3.21 ratio, but you aren't getting same "benefit" for fuel mileage as a factory 3.21. so the whole "effective" ratio BS, isn't even worth talking about.

Yes, a larger tire will give better fuel mileage on the highway because it drops the RPM’s. (Compared to an equivalent smaller tire).

Ram clearly designed the 1500 for optimal performance with 32” tires, as evidenced by the three different tire sizes offered non-Rebel/TRX

275/65/18 (32.08”)
275/55/20 (31.90”)
285/45/22 (32.20”)

 

[lkjk]

Yes, a larger tire will give better fuel mileage on the highway because it drops the RPM’s. (Compared to an equivalent smaller tire).

in a bubble, with no other changed variables that would be true, however in real life you would be fighting decreased aerodynamics, heavier (more rotational mass), potentially more friction if you have a wider tire/bigger contact patch.

I get how the math works for "effective gear ratio", but that's only one variable of many.

Having increased tire size on every vehicle I've owned in the past ~15 years, I've never not had a decrease in MPGs.

 

[Mountain Whiskey]

Depends on how you want to look at it really. You could put on larger tires that "effectively" give a 3.92 geared Ram a 3.21 ratio, but you aren't getting same "benefit" for fuel mileage as a factory 3.21. so the whole "effective" ratio BS, isn't even worth talking about.

HSKR R/T is right. None of this is worth talking about. Understand that it is not just rpm but the effort it takes to move the larger heavier tires that anemic 3:21 gears struggle with. Just get the 3:92 gears so you will be satisfied with your truck and not have to hide your shame of owning a truck with 3:21 gears with denial.

 

[onit43]

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.

Bravo. I feel like a learned something today. Thanks for sharing

 

[5thGenLoco]

HSKR R/T is right. None of this is worth talking about. Understand that it is not just rpm but the effort it takes to move the larger heavier tires that anemic 3:21 gears struggle with. Just get the 3:92 gears so you will be satisfied with your truck and not have to hide your shame of owning a truck with 3:21 gears with denial.

The differential gearing isn't giving the entire picture. Gotta love new transmissions with their lower initial gearing, multiple gear positions, and high final drive ratios.

 

[Mountain Whiskey]

The differential gearing isn't giving the entire picture. Gotta love new transmissions with their lower initial gearing, multiple gear positions, and high final drive ratios.

I know. I just see a lot of guys get real worked up on this thread and it is fun to troll it a bit.

 

[theblet]

3.21 works great. I am able to tow a 16 ft heavy trailer no problem, but still get good mpgs

 

[RamLife]

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.

What would be a better gear ratio if you plan on lifting your truck and putting 35’s on? The 3.21,3.92 or the 3.55?

 

[silver billet]

What would be a better gear ratio if you plan on lifting your truck and putting 35’s on? The 3.21,3.92 or the 3.55?

Go with the 3.92. The 3.55 is not available with the hemi.

 

[RamLife]

Go with the 3.92. The 3.55 is not available with the hemi.

What’s your opinion on etorque or no etorque?

 

[silver billet]

What’s your opinion on etorque or no etorque?

I don't have etorque and have no experience with it. I think the general consensus on this forum is that if you find a truck you like and the price is right, then get it whether or not it comes with etorque. Some here really love it, and there doesn't appear to be any major issues with the system after several years now. In terms of towing, it doesn't gain you any towing power, but the weight of the system (battery etc) will come off your available payload.

 

[Mountain Whiskey]

3:92 is an easy answer. It is far better all around.

Etotque is a split opinion. If you are a guy that keeps a vehicle forever and runs it until it dies, don't get an etorque as it is expensive wear parts. If you trade every two to four years, give it a go. It should last that long with no troubles.

While you are looking, if you check out limited slip vs electronic locking, e lockers are great for off road. They truly lock and only work at low speed. Limited slip is great for the road as it is full time.

 

[firedfly]

What’s your opinion on etorque or no etorque?

We had the same question before we ordered. We ended up test driving both back to back to see if there was much difference. In our test drive, it was immediately obvious that etorque had much smoother gear changes.

We ordered the etorque.

 

[theblet]

I wanted 3.21 gears for mpg and no ET after I looked up the replacement cost of the parts. simpler is better for me so I took the .5 mpg hit.

 

[IvoryHemi]

What’s your opinion on etorque or no etorque?

It was an easy pass for me.

I will keep my truck past 80k so I didn’t want a $1,500 battery or $900 generator.

 

[Idahoktm]

What’s your opinion on etorque or no etorque?

I drove 3 Ram trucks when I was shopping. They all had 3.92 gears but the second one had eTorque. That was the only Ram that I didn't like. I don't know how to describe it but it felt draggy on deceleration...like it had additional engine braking. During acceleration, it felt the same. That sealed the deal for me...no eTorque.

 

[2020Larrie]

3.9 gearing with etorque for me last 20,000 miles in 11 months. Regularly in the mountains of northern Az, no issues.

No doubt better in town and in the hills than lower geared trucks. I am also pulling full 12,000lbs of gravel a few times a month up 7% grades without issue. I get 19-21 MPG city highway (not pulling gravel). On perfectly flat highway for 100 miles I did see 22.

Etorque is an oddity, mine has it because it was the only one available with the options I wanted. They say it helps mileage 1-2mpg. The air suspension squat gives it another 1-2mpg, maybe. The decel on slowing is something to get used to but not an issue. I also do not plan to keep this more than 4 years, it was bought to move and travel then return to a jeep model. I think the engine shutdown at lights gets it best bang for buck fuel saving and the etorqu makes pulling away extremely smooth. Wifes jeep with etorq is not as smooth.

I really wanted a 4x4 model but settled for the sport with 4x4, and limited slip vs lockers. I'm happier with LS than lockers now that I get into the LS a few times a month. Seamless.

Sorry, go for the 3.9.