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!

  • We have been battling an on and off issue with registration verification emails not being sent out with certain users. We have finally got the issue resolved so if you have tried registering for the site but have not received a verification email, just request another verification email when you log in and you should now receive it. If you still don't receive the verification email, send us an email at [email protected] and we will get it resolved for you. Thanks!

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

Jack

Active Member
Joined
Mar 1, 2019
Messages
43
Reaction score
272
Location
Minnesota
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.
 
Last edited:

Neurobit

RAM Sorcerer
Joined
Jul 9, 2018
Messages
5,132
Reaction score
3,754
Location
Texas
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. 3.29, 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. 3.21, 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 - 3.29 = 3.21 - 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 that 3.21 is better at towing: 31-38 MPH, 48-57 MPH,
AND
6) Speed range that 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 with 3.21 might never reach the desired speed uphill within the 58-70 MPH (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 explanation, although it will not end the debate, as folks feel they need to defend their purchases to the death regardless of facts. :unsure:
Thanks for putting the time and effort on this write-up. Should be a sticky.

Cheers,
 

Gitter

Well-Known Member
Joined
Jan 24, 2019
Messages
221
Reaction score
308
Location
Texas
Thanks for the post. I appreciate the breakdown as it gives me more details into how the truck is being geared (and programmed) for the different uses. It also helps confirm the slightly different driving characteristics I felt while test driving each truck.
 

Jack

Active Member
Joined
Mar 1, 2019
Messages
43
Reaction score
272
Location
Minnesota
Great explanation, although it will not end the debate, as folks feel they need to defend their purchases to the death regardless of facts. :unsure:
Thanks for putting the time and effort on this write-up. Should be a sticky.

Cheers,

Thanks! That's true, people seek evidences that support their own beliefs. For 3.92 lovers here, I list the exact reason why 3.92 is better for towing, and for 3.21 lovers, I list exact reason why 3.21 is better for fuel economy. There's something for everyone. Personally, I am leaning 3.21 because I don't plan on owning a boat over 5000lbs, and all the 8000lb and above travel trailers require a 5th wheel found in HD trucks. To be honest, I would most likely be hauling air 99% of the time. If I ever own a boat over 10000lbs, I would probably have enough money laying around for a F350 Tremor. I'm sure many of you with horses or heavy construction equipment would benefit greatly from 3.92. I'm just an engineer chained to a desk.
 

Jack

Active Member
Joined
Mar 1, 2019
Messages
43
Reaction score
272
Location
Minnesota
Thanks for the post. I appreciate the breakdown as it gives me more details into how the truck is being geared (and programmed) for the different uses. It also helps confirm the slightly different driving characteristics I felt while test driving each truck.
Glad I can help. 3.21's highway "sloppiness" is not a bug but a feature, for real.
 

Maconi

Active Member
Joined
Jun 9, 2018
Messages
163
Reaction score
103
I feel like a lot of people like the 3.92 for that 0-30 towing gear. It gives you better acceleration off the line which makes the truck drive a bit more sporty. That may be why some people call the 3.21 "slow" or "sluggish" (even though there isn't THAT much difference).
 

Jack

Active Member
Joined
Mar 1, 2019
Messages
43
Reaction score
272
Location
Minnesota
I feel like a lot of people like the 3.92 for that 0-30 towing gear. It gives you better acceleration off the line which makes the truck drive a bit more sporty. That may be why some people call the 3.21 "slow" or "sluggish" (even though there isn't THAT much difference).
From a stop, the 3.92 will enter the engine power band quicker, and is more likely to kick the tires loose. Overall acceleration should be very similar without load. Overall, for your transmission to maximize engine output, it needs to maintain a final drive ratio that puts your engine's rpm at where the peak output is. It's especially important for heavy towing since situations require you to keep engine at peak output to maintain speed going up an incline.
 
Last edited:

mikeru82

5thGenRams Supporter
Site Supporter
Joined
Nov 18, 2019
Messages
3,272
Reaction score
2,527
Great information. I wasn't aware the two were mated with different transmissions.
 

TruckDriver

Ram Guru
Joined
Dec 18, 2018
Messages
640
Reaction score
398
Location
California
From the list below, we can see that gears 3-7 in 3.21 matches gears 4-8 in 3.92:

Thanks for all the data! As a fellow engineer I had to put it in a table.. too much text for my eyes to focus on haha.

Good statements overall, all around the 3.92 has higher ratios but in some gears, near-as-makes-no-difference for the seat-dyno.

1594143543070.png
 
Last edited:

Jack

Active Member
Joined
Mar 1, 2019
Messages
43
Reaction score
272
Location
Minnesota
Thanks for all the data! As a fellow engineer I had to put it in a table.. too much text for my eyes to focus on haha.

Good statements overall, all around the 3.92 has higher ratios but in some gears, near-as-makes-no-difference for the seat-dyno.

View attachment 44185
Yeah I was too lazy to open up excel or matlab
 

Users who are viewing this thread

Top