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

Idahoktm

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Why do you bother asking this type of question? There is no answer I can give that will make you change your mind.

If I say: "Yes I've owned both" you'll just say "well your experiences aren't representative"
If I say: "No I've never owned either" you'll say "there you go, you can't possibly know the fuel economy of either one"
If I say: "I've driven both" you'll say "driving isn't the same as long term ownership"
If I say: "I've owned and/or driven both" you'll say "you need to drive the one longer to form a better idea"

... and so on.

You made up your mind without facts, so facts cannot be used to change your mind. Just move on, argue with the wind, because I'm done.
So, to summarize:

Silver Billet is the only one stating facts and everyone else is full of 💩.

Mods, please close this thread...he hath spoken. 🤣
 

silver billet

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So, to summarize:

Silver Billet is the only one stating facts and everyone else is full of 💩.

Mods, please close this thread...he hath spoken. 🤣

Oh no, keep it open and feel free to circle jerk around. I'm headed out as I simply don't have the stud credentials to hang with you 3.92 guys.
 

HSKR R/T

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Why do you bother asking this type of question? There is no answer I can give that will make you change your mind.

If I say: "Yes I've owned both" you'll just say "well your experiences aren't representative"
If I say: "No I've never owned either" you'll say "there you go, you can't possibly know the fuel economy of either one"
If I say: "I've driven both" you'll say "driving isn't the same as long term ownership"
If I say: "I've owned and/or driven both" you'll say "you need to drive the one longer to form a better idea"

... and so on.

You made up your mind without facts, so facts cannot be used to change your mind. Just move on, argue with the wind, because I'm done.
Sounds like a lot of excuses to avoid answering the question cause you know the only "facts" you have, that apply here, are your experiences with your current 3.21 geared truck, and any numbers you have tried to throw out to compare to 3.92 truck are hypothetical or theoretical. It saying there isn't a difference, cause it's obvious there will be. Just not as dramatic as you and others are trying to make it seem
 

Idahoktm

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Sounds like a lot of excuses to avoid answering the question cause you know the only "facts" you have, that apply here, are your experiences with your current 3.21 geared truck, and any numbers you have tried to throw out to compare to 3.92 truck are hypothetical or theoretical. It saying there isn't a difference, cause it's obvious there will be. Just not as dramatic as you and others are trying to make it seem
He would make a great politician. 😉
 

Mountain Whiskey

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TNRamGuy

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Ok, so after 910 posts we are sooo close to having a definitive answer on "what's the best gearing? " from what I can tell the last 2 outstanding items that need clarification are:
1. How much faster is it with a new cabin air filter?
2. Do the tires rub and will I need a level?

Lmfao
 

HSKR R/T

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Ok, so after 910 posts we are sooo close to having a definitive answer on "what's the best gearing? " from what I can tell the last 2 outstanding items that need clarification are:
1. How much faster is it with a new cabin air filter?
2. Do the tires rub and will I need a level?

Lmfao
The answer is simple. The best gear set is the one you choose to have on your truck. Because it's your choice
 

Omiester2

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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 Jack!
 

Mountain Whiskey

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Ok, so after 910 posts we are sooo close to having a definitive answer on "what's the best gearing? " from what I can tell the last 2 outstanding items that need clarification are:
1. How much faster is it with a new cabin air filter?
2. Do the tires rub and will I need a level?

Lmfao
Those answers are actually obvious.

1. As long as you have 3:92 gearing it is a documented fact in controlled lab experiments, hypothetical computer modeling, on the track, on the road and in the depths of people's own minds that a new cabin air filter you will outrun time as you outpace the light trailing after you.

2. Since a truck with 3:21 gears can barley handle the tires stolen off a Toyota Yaris that come stock, they have no troubles rubbing. These will easily make it to the Tupperware party or baby shower.

Now if you wish to run truck tires you should have a lift and real gearing. Go 3:92. Hit the strip club and park on top of a Prius.
 
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TNRamGuy

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Those answers are actually obvious.

1. As long as you have 3:92 gearing it is a documented fact in controlled lab experiments, hypothetical computer modeling, on the track, on the road and in the depths of people's own minds that a new cabin air filter will come close to getting you will outrun time as you outpace the light trailing after you.

2. Since a truck with 3:21 gears can barley handle the tires stolen off a Toyota Yaris that come stock, they have no troubles rubbing. These will easily make it to the Tupperware party or baby shower.

Now if you wish to run truck tires you should have a lift and real gearing. Go 3:92. Hit the strip club and park on top of a Prius.
#1, so, with 3.92 I can effectively get to my destination before I even decide where I want to go. Sweet, I knew I got them for a reason!!
 

Mountain Whiskey

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Prius at the strip club??
Well, you have a point. It might belong to one of the girls but then it just wouldn't be right to park on top of it.

Hey do you think one of the FCA engineer geeks saw this and thought up the electric Ram Truck?
 

Rebelguy2020

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My city driving right now View attachment 107411

As for the complaining, look in the mileage thread, their gears are listed in their signatures and the vast majority are 3.92 but you absolutely don't have to take my word for it, browse those threads and see for yourself.


And before you move the goal posts View attachment 107412

I have always averaged 17-18 mpg in the city and I don't drive slow. I drive 5 to 10 mph over the speed limit regularly
I see in your signature that you have a 4X2 and it can’t be an equal comparison with 4X4, the window sticker will show a slight advantage for the 4X2 but in reality the 4X4 will have an even bigger difference (meaning worst mpg) than the window sticker mpg.
 
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