5thGenRams Forums

Register a free account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members through your own private inbox!

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

bcso0416

Member
Joined
Nov 2, 2020
Messages
13
Reaction score
2
Location
East Tennessee
Would it be possible to list the values using the 3.55 Gear? I have a 2020 4X4 V-6 e-torque with the 3.21 gearing and enjoy the around town mpg of (21). The window sticker says 19 city and 24 hwy. The highway mpg is not any better or worse, and it has 15000 miles on it, so it should be broke in. Anyway, the towing limits on what I have is showing max at 6800 lbs. with an advisory from RAM not to go over 6400 lbs. According to the RAM web site and brochures, if it had the 3.55 gear, the towing would have been 1,000 lbs. more, 7800 lbs. with an advisory of 7400 lbs. It is also saying that the mpg would stay at 19 city and 24 hwy with the 3.55 gearing and the only difference would be better towing. If all this is true and I had known this, I would have gone for that combo instead. Having the same MPG and better towing is a no brainer. With the 3.55 gear, I might even get better hwy mpg who knows. I may try and find another truck with that combination as I would like to have the extra towing and the ability to keep engine at peak output to maintain speeds going up inclines, especially on the interstate. Your information is very helpful and I along with others I'm sure appreciated it very much. Thanks
 

SnowBlaZR2

Fuel Economy Champion
Site Supporter
Joined
Oct 17, 2021
Messages
2,001
Reaction score
2,691
Location
FL
Would it be possible to list the values using the 3.55 Gear? I have a 2020 4X4 V-6 e-torque with the 3.21 gearing and enjoy the around town mpg of (21). The window sticker says 19 city and 24 hwy. The highway mpg is not any better or worse, and it has 15000 miles on it, so it should be broke in. Anyway, the towing limits on what I have is showing max at 6800 lbs. with an advisory from RAM not to go over 6400 lbs. According to the RAM web site and brochures, if it had the 3.55 gear, the towing would have been 1,000 lbs. more, 7800 lbs. with an advisory of 7400 lbs. It is also saying that the mpg would stay at 19 city and 24 hwy with the 3.55 gearing and the only difference would be better towing. If all this is true and I had known this, I would have gone for that combo instead. Having the same MPG and better towing is a no brainer. With the 3.55 gear, I might even get better hwy mpg who knows. I may try and find another truck with that combination as I would like to have the extra towing and the ability to keep engine at peak output to maintain speeds going up inclines, especially on the interstate. Your information is very helpful and I along with others I'm sure appreciated it very much. Thanks
3.55.PNG
 

jsmith7364

New Member
Joined
Jan 9, 2022
Messages
1
Reaction score
0
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 info. And this explains why I get better mpg at 77-81 mph with the 3:21 gears and slightly larger tires with the EcoDiesel. I need to lower that down some and try to gain a little more performance when towing so i will probably change the gears to 3:92 or 4:10. Match that with the loss of a towing gear down low but with the extra overdrive on top and it should workout much better for how I use the truck.
 

WXman

Ram Guru
Joined
Feb 29, 2020
Messages
899
Reaction score
616
Location
Kentucky, USA
I picked up a Laramie yesterday and this one has 3.21s. Was shocked when I got home and plugged the VIN into the Ram site and saw that this truck is rated 8,100 towing. Ouch! That's only 400 lbs more than a Jeep pickup I had a few years ago.
 

theblet

Spends too much time on here
Joined
Feb 8, 2021
Messages
2,118
Reaction score
1,831
I picked up a Laramie yesterday and this one has 3.21s. Was shocked when I got home and plugged the VIN into the Ram site and saw that this truck is rated 8,100 towing. Ouch! That's only 400 lbs more than a Jeep pickup I had a few years ago.
Of you’re gone tow heavy, a camper, etc 3.92 are really the way to go
 

GMetal

Member
Joined
Jul 20, 2022
Messages
20
Reaction score
11
3.92 gear and 11mpg. Its a truck that pulls heavy things and doesn't stink like diesel.
 

Users who are viewing this thread

Top