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

[theblet]

Yea, just don't cheer for your driver on this forum. Let's Go (insert driver's name here) if forbidden by the woke censors.

Those we do not speak of! It’s like we’re in the village.

 

[Mountain Whiskey]

Those we do not speak of! It’s like we’re in the village.

Right on! I have been trying to remember the name of that movie all week!

 

[theblet]

Right on! I have been trying to remember the name of that movie all week!

Haha! And they had a secret agenda and a monster behind the curtain just like we do today!

 

[Pilk]

I'm getting my order ready for a Laramie Midnight Edition and I was going 3.21 then I wondered whether the 22"x9" 285/45r22's would weigh enough more to consider the 3.92. Is there a source for checking the weight of the various wheel/tire options?

 

[Idahoktm]

I'm getting my order ready for a Laramie Midnight Edition and I was going 3.21 then I wondered whether the 22"x9" 285/45r22's would weigh enough more to consider the 3.92. Is there a source for checking the weight of the various wheel/tire options?

The stock tires on the Night Edition trucks are an XL rated tire. They should weigh around 50lbs each, which is fine with the 3.21 gears. If you plan on getting bigger, LT type tires, you might want to consider 3.92's. I'm assuming you're getting the Hemi??

 

[theblet]

I'm getting my order ready for a Laramie Midnight Edition and I was going 3.21 then I wondered whether the 22"x9" 285/45r22's would weigh enough more to consider the 3.92. Is there a source for checking the weight of the various wheel/tire options?

the 3.21s wont have a problem. Get the 3.92s if you are going to tow often or plan on oversized tires. Otherwise your cruising rpm will be higher. At least that’s how I approached it.

 

[Mountain Whiskey]

I'm getting my order ready for a Laramie Midnight Edition and I was going 3.21 then I wondered whether the 22"x9" 285/45r22's would weigh enough more to consider the 3.92. Is there a source for checking the weight of the various wheel/tire options?

This site has all kinds of tire information if you look around.

https://tiresize.com/

Good old Discount Tire lists tire weights on their website too.

In the end, just get the proper 3.92 gears and be happy. Otherwise you will be like the others on here who settled for second best and spend all their time trying to justify why their poor choice was right for them.

I mean, 3.21 gears with a Hemi is like putting Aristocrat whiskey in a Pappy Van Winkle bottle and bragging to your buddies about your great liquor.

 

[Ramroo]

I'm getting my order ready for a Laramie Midnight Edition and I was going 3.21 then I wondered whether the 22"x9" 285/45r22's would weigh enough more to consider the 3.92. Is there a source for checking the weight of the various wheel/tire options?

You can just go to the tire manufacturer websites and get all the size and weights there. There are lots of differing opinions on this subject. I suggest read all you can stand. No way I’m going to make a suggestion because, like I said, there are lots of opinions. However, if you can find a 3.92 and a 3.21 truck to test drive, do that. Take them on good tests drives, in town, expressway, etc. Stop and go to hauling ***.

 

[Ramjack]

Yea, just don't cheer for your driver on this forum. Let's Go (insert driver's name here) is forbidden by the woke censors.

Let's go crazy-as$ senile old fart, let's go !! (See?? No driver name mentioned.)

 

[Mountain Whiskey]

Let's go crazy-as$ senile old fart, let's go !! (See?? No driver name mentioned.)

I love it!

 

[Ramjack]

Do-dee-do-do, do-dee-do . . . <Jeopardy theme>

 

[SnowBlaZR2]

 

[jonese]

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.

Thanks so much for the scientific approach, it's really appreciated.

A question for you regarding our circumstances. We live in Colorado and go to the mountains quite a bit. However, we rarely tow anything and occasionally do some moderate off-roading (imagine elk hunting). Even when we do tow, it would not approach the 8,000 lbs of the 3.21 axle ratio limits. Our typical load in the pickup would be four people, two labs, and some camping gear of a few hundred pounds in the bed. Also, where we head in the mountains does go over some mountain passes of about 11,000 feet elevation and typical speeds would be in the range of 45-65 mph (45 mph over the passes, and 65 mph elsewhere).

Given that there is so much overlap in the final ratios (i.e 3.21 3rd gear matches up with 3.92 4th) would we benefit from the 3.92 axle ratio? Thanks in advance.

 

[Mountain Whiskey]

Thanks so much for the scientific approach, it's really appreciated.

A question for you regarding our circumstances. We live in Colorado and go to the mountains quite a bit. However, we rarely tow anything and occasionally do some moderate off-roading (imagine elk hunting). Even when we do tow, it would not approach the 8,000 lbs of the 3.21 axle ratio limits. Our typical load in the pickup would be four people, two labs, and some camping gear of a few hundred pounds in the bed. Also, where we head in the mountains does go over some mountain passes of about 11,000 feet elevation and typical speeds would be in the range of 45-65 mph (45 mph over the passes, and 65 mph elsewhere).

Given that there is so much overlap in the final ratios (i.e 3.21 3rd gear matches up with 3.92 4th) would we benefit from the 3.92 axle ratio? Thanks in advance.

Yes.

 

[Ramjack]

Thanks so much for the scientific approach, it's really appreciated.

A question for you regarding our circumstances. We live in Colorado and go to the mountains quite a bit. However, we rarely tow anything and occasionally do some moderate off-roading (imagine elk hunting). Even when we do tow, it would not approach the 8,000 lbs of the 3.21 axle ratio limits. Our typical load in the pickup would be four people, two labs, and some camping gear of a few hundred pounds in the bed. Also, where we head in the mountains does go over some mountain passes of about 11,000 feet elevation and typical speeds would be in the range of 45-65 mph (45 mph over the passes, and 65 mph elsewhere).

Given that there is so much overlap in the final ratios (i.e 3.21 3rd gear matches up with 3.92 4th) would we benefit from the 3.92 axle ratio? Thanks in advance.

At that elevation, you'll lose more than 30% power. A 3.92 rear would be helpful.

 

[Mountain Whiskey]

Or at sea level it will be helpful too.

 

[Mirowpl]

Wrong wrong wrong. I live at sea level or close to it at 16ft above sea level. My 3.21 gears are the best.

 

[Mountain Whiskey]

They are the best right up until the truck actually has to move. I guess if you keep Prius tires on it and don't haul more than your average trip to Krogers you can get by. Avoid Costco as that grocery run might be too much for the 3.21s!

 

[Mirowpl]

you will need all of that 33 gallon fuel tank

 

[Mountain Whiskey]

you will need all of that 33 gallon fuel tank

I know. Have to fill up before it hits $5.