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

Bigdumps

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What? 2 weeks has past and no one with 3:21 gears has been on here bragging how good their milage is while old Datsun pickups loaded with landscaping equipment are passing them asking if they need any help to get rolling.

We can't let the thread die. It hasn't cracked 1000 posts yet!
As someone newer here.
I've had a 2012, and a 2015 with the 3.92. best power and fuel economy out of any truck I've owned. My 2015 sport would get roughly 500+miles per 33 gallon tank.
My 2020 had the 3.21(wasn't focused when buying, switched dealers after that)with the 27 gallon? tank maybe would be lucky to crack 325 miles. that thing sucked. I seen no increased fuel economy, and I drive like a senior 90% of the time.
Couldn't wait to get rid of that truck. I raced them both side by side the shortly after I got my 2020. My 2015 dusted my 2020, no contest.

Got my 2022 Sport/Laramie GT coming in with the 3.92.

The 3.21 shouldn't even be offered.
 

ferraiolo1

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Weird, I can get 500ish out of my 27gal tank and 3.21s on the highway as shown two posts above yours.

Gearing options are just that, an option for people who want options.


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KWKSLVR

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As someone newer here.
I've had a 2012, and a 2015 with the 3.92. best power and fuel economy out of any truck I've owned. My 2015 sport would get roughly 500+miles per 33 gallon tank.
My 2020 had the 3.21(wasn't focused when buying, switched dealers after that)with the 27 gallon? tank maybe would be lucky to crack 325 miles. that thing sucked. I seen no increased fuel economy, and I drive like a senior 90% of the time.
Couldn't wait to get rid of that truck. I raced them both side by side the shortly after I got my 2020. My 2015 dusted my 2020, no contest.

Got my 2022 Sport/Laramie GT coming in with the 3.92.

The 3.21 shouldn't even be offered.
If you were getting 12mpg then there was something mechanically wrong with that truck.
 

Mountain Whiskey

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As someone newer here.
I've had a 2012, and a 2015 with the 3.92. best power and fuel economy out of any truck I've owned. My 2015 sport would get roughly 500+miles per 33 gallon tank.
My 2020 had the 3.21(wasn't focused when buying, switched dealers after that)with the 27 gallon? tank maybe would be lucky to crack 325 miles. that thing sucked. I seen no increased fuel economy, and I drive like a senior 90% of the time.
Couldn't wait to get rid of that truck. I raced them both side by side the shortly after I got my 2020. My 2015 dusted my 2020, no contest.

Got my 2022 Sport/Laramie GT coming in with the 3.92.

The 3.21 shouldn't even be offered.
Yea, if you read the hundreds of posts, you will find some 3:21 guys who brag they get better milage than a Tesla or at least a Prius. Many think milage has only to do with mathematic equations depicting rpm with no reguard given to how hard the motor works to maintain those rpms.

Drive a smaller motor and this becomes even more apparent. Go on the jeep forums a bit and you will see guys going up to crazy 4.56 and up seeing milage improve because the motor is not lugging along to try and keep up when they put real tires on.

But you know, some just have to justify their decisions, even when wrong.
 

Bigdumps

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Weird, I can get 500ish out of my 27gal tank and 3.21s on the highway as shown two posts above yours.

Gearing options are just that, an option for people who want options.


Sent from my iPhone using Tapatalk
Lucky. I even put 15000km on it from September 1-November 10. Primarily highway during bow season. My "eco" hardly illuminated. Found it shifting between 7-8 in winds and hills.(Northern, and western Alberta)
To each their own. but for me, Never again.
 

c3k

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Yea, if you read the hundreds of posts, you will find some 3:21 guys who brag they get better milage than a Tesla or at least a Prius. Many think milage has only to do with mathematic equations depicting rpm with no reguard given to how hard the motor works to maintain those rpms.

Drive a smaller motor and this becomes even more apparent. Go on the jeep forums a bit and you will see guys going up to crazy 4.56 and up seeing milage improve because the motor is not lugging along to try and keep up when they put real tires on.

But you know, some just have to justify their decisions, even when wrong.
LOL...

Not arguing...but. ;) (All the below is referencing my ecodiesel)

I've got a 3.21 and swapped from OEM tires (32") to 34" 10-ply. I went from 48lb tires to 68lb tires. With OEM, my around-town was ~25mpg. Highway was 30+. (Unless I was driving 80 or more.) If I went about 75mph, I'd get a consistent 28mpg, rolling hills.

Now, with the heavier tires and larger diameter, the shifting occurs at different speeds (of course: the diesel won't stay engaged below ~1,000rpm: it'll downshif). At 73mph, I'm seeing 1550rpm (8th gear) and getting ~25mpg (cruise control engaged and driving for an hour or more).

I did not get it for the mileage. But, it helps. A lot.

My numbers are just for info. Take them as they are and do with them as you will...
 

Mountain Whiskey

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LOL...

Not arguing...but. ;) (All the below is referencing my ecodiesel)

I've got a 3.21 and swapped from OEM tires (32") to 34" 10-ply. I went from 48lb tires to 68lb tires. With OEM, my around-town was ~25mpg. Highway was 30+. (Unless I was driving 80 or more.) If I went about 75mph, I'd get a consistent 28mpg, rolling hills.

Now, with the heavier tires and larger diameter, the shifting occurs at different speeds (of course: the diesel won't stay engaged below ~1,000rpm: it'll downshif). At 73mph, I'm seeing 1550rpm (8th gear) and getting ~25mpg (cruise control engaged and driving for an hour or more).

I did not get it for the mileage. But, it helps. A lot.

My numbers are just for info. Take them as they are and do with them as you will...
Haha! Smokers don't count! That's a different game. 😄
 
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Bigdumps

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If you were getting 12mpg then there was something mechanically wrong with that truck.
Best case, was getting roughly 15mpg. Maybe some people got unicorns that got a unheard of 25 mpg. Which is roughly 9L/per100km. If that was my truck, I would call Chrysler and let them know they created a miracle. 😂 Trucks burn that when they're sleeping.
 

Hasti17

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Best case, was getting roughly 15mpg. Maybe some people got unicorns that got a unheard of 25 mpg. Which is roughly 9L/per100km. If that was my truck, I would call Chrysler and let them know they created a miracle. 😂 Trucks burn that when they're sleeping.
25mpg is a true and common phenomena, its the "if you want good mileage buy a diesel and pick whatever rear you want" if you dont tow? 3.21, 25-30mpg, towing a part of life? 3.92, 23-29mpg.
 

Rebman24

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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.
I have 3.21 ad tow a 6700lb TT no problem at all. Been on hills, etc.. Unless u are towing over 7000k lbs or tow all the time, , then the 3.21i is just fine for those who have it..
3.92 is also much harder to find now too..
 

HSKR R/T

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I have 3.21 ad tow a 6700lb TT no problem at all. Been on hills, etc.. Unless u are towing over 7000k lbs or tow all the time, , then the 3.21i is just fine for those who have it..
3.92 is also much harder to find now too..
I mean, for that matter, the 3.92 is "just fine" for those who have it when it comes to fuel economy.
 

Mirowpl

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I have 3.21 gears, had it on my 2009. i do tow my boat and other things but not ona daily basis. I actually think it’s the best for most. I think it is so funny all the Crap on this thread. If I really needed the 3.92, I would have gotten a truck with that gear, but I don’t so the 3.21 is just fine. Here was a recent mpg No complaints from me.
 

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Mountain Whiskey

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Many here are right. You don't need 3:92 gears in a 5.7 Hemi truck.

I mean, plenty of guys are fine with mediocre, run of the mill trucks that just barely get the job done. Some guys are perfectly fine never having anything better for themselves. Some guys never get out of the slow lane. Some are slow witted mouth breathers. Some guys have neckbeards and wear Berkenstock sandals. Some guys even buy trucks with 3:21 gear ratios.

It is amazing the lackluster people that exist in this world just to exist.
 

jimchi

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i’m surprised this thread has gone so long. my understanding of the consensus is:
  • 3.92: tow often, want fast off-the-line numbers, or are planning to get big tires
  • 3.21: lots of highway driving, or everyone else
honestly some of the more recent posts are really just people trying to justify their own insecurities. get whatever suits your need. imho, the real decision is open vs. lsd vs. locking
 

Rebelguy2020

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i’m surprised this thread has gone so long. my understanding of the consensus is:
  • 3.92: tow often, want fast off-the-line numbers, or are planning to get big tires
  • 3.21: lots of highway driving, or everyone else
honestly some of the more recent posts are really just people trying to justify their own insecurities. get whatever suits your need. imho, the real decision is open vs. lsd vs. locking
You are right about the 3.21 having better mpg on the highway at higher speeds (above 60 mph) and if you don’t tow a lot or you don’t need the maximum tow capacity of a 1/2 ton truck.

In my case, my Rebel (diesel) has the 3.92, the maximum speed limit here in Northern Ontario Canada is 90 kph or 55 mph, I would have to drive 4 hours south to get to a multi lane freeway.
The 3.92 advantage for me is that my diesel Rebel has 480 ft/lbs of torque at 1600 rpm so when I drive at 90 kph or 55 mph on the highway my truck’s rpm is at 1600 rpm therefore I get the maximum torque and best fuel efficiency, the biggest advantage is towing at that speed. I do realize that the 3.21 would be close to 1600 rpm at 55 mph in 7th gear, so going faster it would be at an advantage because of the 8th gear.

The 3.92 has a slight advantage in city driving, it takes less effort to get the truck moving from a stop or a red light, so if someone barely takes their truck on the highway the 3.92 is better, the advantage is even greater if you tow in town.
 

Mountain Whiskey

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It's a great thread. Ssooooo much good information after the first page of posts. Hopefully in reading through it you have gleaned valuable knowledge.

In a nutshell, those that have 3:21 gears realize that the truck is anemic and are covering for thier regret by swearing the only factor in gas milage is rpm. By believing this farce they convince themselves that they get great mileage. Since thier daily driver is a 4 cylinder minivan they think they have power now to.

The guys with 3:92 geared trucks know they made the right choice because they understand trucks. They put on larger tires and know what milage to expect from a V8.

Simple.
 

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