3.21. vs 3.92 gear

[devildodge]

To me, as long as you are driving a speed which the transmission would use 2 - 7 for, there should be no difference between the two trucks.

Very confused by this comment. It was said that a 3.92 in 8th is the same as a 3.21 in 7th. This really only works here.

1 thru 6 are direct drive...7 and 8 overdrive.

3.92 will produce the torque effect more than 3.21 in all gears.

I have a HD with 6.4l and 3.73. 2nd to first is a huge jump, and with 4.10 I could hold second longer than 3.73 instead of having that annoying downshift.

So, here in PA we have some very steep roads. 3.92 in second will have more power than a 3.21 in 2nd. And that is the area that the tow ratings come into play. That 3100 extra lbsthe 3.92 is rated for...that is where it get that extra rating.

Also, 3.92 will help with engine braking going down the hill. Hence the 3100lb more GCWR.

Once again...the only non opinion reason to get the 3.92 over the 3.21 is the 3100lb extra rating.

But, then, we also know how guys feel about the rating.

So, it is opinion and your own opinion is all that matters.

 

[riccnick]

The 3.92 will provide approximately a 22% increase in multiplied power to the wheels over the 3.21 rear end in any and all gears, when comparing gear to gear output.

However, when considering the overall drive ratio in each gear and combined with each rear end option, there can be an overlap of applied torque at the wheels when different gears are selected. The example of 7th gear and 3.21 is the same as 8th gear and 3.92 can be applied to other ratio combo's as well. Is 2nd gear with the 3.21 close to 3rd with the 3.92? My bet is each ration step-down is pretty close, with the obvious exclusions being 1st gear in either truck.

I'm going to find and calculate the drive ratio's and put them in a chart, we'll be able to see the overlap if it exists for other gears, or at least see how close they get to being the same.

 

[NDanecker]

Please explain how you get more gears to play with with the 3.92? These are both 8 speeds, and once both trucks are between 2 and 7, there is no difference.

You get more torque to the wheels with 3.92 vs. 3.21 for the same gear ratio. Plus the gears will be spaced closer together so you don't have to over rev to jump to the next gear. For example, you say 7th with 3.21 is same as 8th with 3.92. If that statement is correct (I haven't' confirmed the ratios) then you have 1 extra gear with the 3.92s (considering both trucks are running same speed and same engine rpms).

It's all about torque multiplication. The lower (numerically higher) gear ratio multiplies the torque. Using round numbers, if your truck is producing 100lbft of torque and you have the 3.92 RAR, ignoring the rest of the drivetrain, your truck is putting 392lbft of torque to the ground. A 3.21-equipped truck would have 321lbft of torque going to the ground in the same scenario. That's about a 20% difference, hence in our simplified example, the truck with 3.21s will be working about 20% harder to go up that hill.

That's how I understand it, someone please correct me if I'm wrong.

You are on the right track. However, once you take the transmission into account the numbers are somewhat different. For example, 8th gear is 1:0.67 so if the engine is putting out 400 ft/lbs of torque you are putting 1050 ft/lbs to the axle with 3.92s and 860 ft/lbs with 3.21. With stock 32" tires and assuming 20% parasitic loss through drivetrain that is 631 lbs and 517 lbs force respectively to the ground.

If I had the free time and care I'd make a spreadsheet for each gear ratio and RPM to show the true output gains with 3.92 gear set over the 3.21 but hey - they are just numbers. Real live experience rules there. LOL

 

[silver billet]

The 3.92 will provide approximately a 22% increase in multiplied power to the wheels over the 3.21 rear end in any and all gears, when comparing gear to gear output.

However, when considering the overall drive ratio in each gear and combined with each rear end option, there can be an overlap of applied torque at the wheels when different gears are selected. The example of 7th gear and 3.21 is the same as 8th gear and 3.92 can be applied to other ratio combo's as well. Is 2nd gear with the 3.21 close to 3rd with the 3.92? My bet is each ration step-down is pretty close, with the obvious exclusions being 1st gear in either truck.

I'm going to find and calculate the drive ratio's and put them in a chart, we'll be able to see the overlap if it exists for other gears, or at least see how close they get to being the same.

That idea of "overlap" is kind of what I'm asking about in a round-about way. The torque from the engine varies as you go up/down the RPMs. So it maybe true that a 3.92 at 2000 RPM's is putting out more torque than the 3.21 at 2000 RPM's, but these transmissions have so many gears that the 3.21 can simply downshift, bump up the revs from 2000 to (say) 2500 or 3000 and then get a similar torque number.

And the fact that these are 8 speeds, means that you don't have to rev very high between shifts, the amount of available gears means we have good RPMs/torque for almost any speed.

I would love to see the chart if you have the energy and time to make one. This all rather abstract right now, but some nice cold hard numbers might clear it up for me anyway.

 

[Gman]

This is all about 'mechanical advantage'.

Mechanical advantage - Wikipedia

en.wikipedia.org

This webpage does a pretty good job of illustrating the differences;
Gear ratio and mechanical advantage

When ever the driver is connected to a larger gear, force is multiplied. The follower rotates slowly compared to the driver but with a greater force.

You don't get something for nothing. If you want better towing, you go for the 3.92. If you want to maximize the number of miles you cover per gallon, go for the 3.21.

 

[riccnick]

You don't get something for nothing. If you want better towing, you go for the 3.92. If you want to maximize the number of miles you cover per gallon, go for the 3.21.

But you get really damn close (I said d-a-m-n, it's not that bad of a word) to the same mechanical advantage when you go one gear lower on a 3.21 truck when compared to a 3.92 truck. You just lose out on the extremes of the ratio spread, and the available ratios you have to choose from to achieve the same task. The other restriction is the engine operating range, if you're maxed out in rev's with a 3.92 truck, the 3.21 truck obviously can't choose a lower gear due to over-revving. So there ARE reasons to get the 3.92, and there ARE reasons why it can tow more, it's just that for 95% of driving, the performance is going to be really damn (d-a-m-n) close



Essentially, at speed, through gears 3-7, you get the same (within less than 10% difference) operating mechanical advantage as a 3.92 in gears 4-8.

 

[Willwork4truck]

Only enthusiasts can debate this for 13+ pages, ha ha. Yep, an oversimplification is: more power for hauling or towing or mountain driving or even racing is the reason to choose 3.92, versus better gas mileage (usually) when empty is 3.21.
I have plenty of admiration for the ZF and the grunt of the hemi, and I prefer better gas mileage since I’ve quit driving/working it hard. If our truck had come with the 3.92 I’d have been happy. It came with the 3.21, I’m still happy...

 

[Willwork4truck]

But you get really **** close (I said d-a-m-n, it's not that bad of a word) to the same mechanical advantage when you go one gear lower on a 3.21 truck when compared to a 3.92 truck. You just lose out on the extremes of the ratio spread, and the available ratios you have to choose from to achieve the same task.

View attachment 28388

Essentially, at speed, through gears 3-7, you get the same (within less than 10% difference) operating mechanical advantage as a 3.92 in gears 4-8.

Thanks for the work. The gearheads will relish examining it in detail.

 

[Gman]

I read on the Internet that the 3.21 is better than the 3.92. (So it must be true)

/THREAD

 

[Diamondback]

Compromise and get the 3.55

 

[Gman]

Compromise and get the 3.55

I think FCA already sold both of them.

 

[Diamondback]

HAHAHAHAHA!!!

 

[KLSU]

What I don't understand(Seriously I don't) is IF there isn't a difference in 3.21 and 3.92 except just in mountains or quicker acceleration then why does a Ram Crew 4x4 with a 3.92 rear end have a tow rating of 11,340 and the same one with 3.21 is rated at 8,240?

 

[riccnick]

What I don't understand(Seriously I don't) is IF there isn't a difference in 3.21 and 3.92 except just in mountains or quicker acceleration then why does a Ram Crew 4x4 with a 3.92 rear end have a tow rating of 11,340 and the same one with 3.21 is rated at 8,240?

Because it's in the extreme scenarios where the 3.92 will be able to perform better. And I'm not just talking sheer acceleration. Engine and trans cooling under load will be much better with the 3.92 when compared to the 3.21. Just because there is "some" overlap in the gears, doesn't mean you can use it when approaching the limits of the truck. That overlap diminishes with more load on the engine, and when the trans is looking for lower gears to get the same amount of work done. Adding an additional 4,000 lbs to the back of the truck is no joke (~8,000lbs vs 12,000 lbs towing capacity), that's a lot of weight, and that's where the differences between the two gear sets will be the most apparent.

 

[devildodge]

What I don't understand(Seriously I don't) is IF there isn't a difference in 3.21 and 3.92 except just in mountains or quicker acceleration then why does a Ram Crew 4x4 with a 3.92 rear end have a tow rating of 11,340 and the same one with 3.21 is rated at 8,240?

Read my many posts on this.

The only real difference between them is GCWR 13900 for 3.21 and 17000 for 3.92

Which is the 3100lb difference.

Hence why the only thing you will find rated different is the max tow rating. Which on the website is completely wrong in 98% of cases.

The rest of the debate is personal preference.

 

[KLSU]

Because it's in the extreme scenarios where the 3.92 will be able to perform better. And I'm not just talking sheer acceleration. Engine and trans cooling under load will be much better with the 3.92 when compared to the 3.21. Just because there is "some" overlap in the gears, doesn't mean you can use it when approaching the limits of the truck. That overlap diminishes with more load on the engine, and when the trans is looking for lower gears to get the same amount of work done. Adding an additional 4,000 lbs to the back of the truck is no joke (~8,000lbs vs 12,000 lbs towing capacity), that's a lot of weight, and that's where the differences between the two gear sets will be the most apparent.

Gotcha, So what you are saying is if you stay under the 8k towing (Which towing more than that on a consistent basis you probably should be 2500 anyway) then there is not much difference correct?

 

[JJRamTX]

What I don't understand(Seriously I don't) is IF there isn't a difference in 3.21 and 3.92 except just in mountains or quicker acceleration then why does a Ram Crew 4x4 with a 3.92 rear end have a tow rating of 11,340 and the same one with 3.21 is rated at 8,240?

I think what you are looking for is wear and tear. The 3.21 will have to work harder at higher RPMs for the engine in a lower gear (Gas wasted) (Transmission wear) (Harder for the Truck to control downhill decents). The 3.21 can do it but is won't be as easy or controlled as the 3.92

 

[riccnick]

Gotcha, So what you are saying is if you stay under the 8k towing (Which towing more than that on a consistent basis you probably should be 2500 anyway) then there is not much difference correct?

There's kind of two different trains of thought that people have been taking in this thread. Either someone says you need a 3.92 to tow anything at all, or someone says the 3.21 has the same transmission so it must be able to tow the full 12k instead of 8.

Neither of those are correct. This is not as black and white as that, in either extreme. I'm in the middle defending both sides of the argument as long as it's within the rated capability of the truck.

Here's what I'm saying. If your truck is rated to tow 8k, then you'll be fine towing 8k or less. If your truck is rated to tow 12k, then you'll be fine towing 12k or less. It's up to YOU to choose the correct truck for YOUR application, and is NOT the forum's responsibility to guide you on what's "OK" when it comes to overloading a truck and using it beyond it's intended capability. That part to me IS pretty black and white. If you know what you need for a weight rating, get the right truck. If you don't know what you need for a weight rating, and buy a truck that ends up not being capable enough for what you want to do, you either need to compromise on what you're trying to tow, or get a more capable truck. It really is that simple.

(The "YOU"'s there are collective you's, not personal, fyi)

 

[Jus Cruisin]

I think what you are looking for is wear and tear. The 3.21 will have to work harder at higher RPMs for the engine in a lower gear (Gas wasted) (Transmission wear) (Harder for the Truck to control downhill decents). The 3.21 can do it but is won't be as easy or controlled as the 3.92

I don't agree with your rpm statement. I don't care what gearing you have, maximum torque for the engine is reached at the same rpm (3500 or so). The transmission is gonna find the big torque #'s when under load. It might be 5th gear at 65mph with a 3.21 or 6th gear with a 3.92. Either way, the truck will be running similar rpm's towing and the 3.21 will turn fewer rpm's empty at similar speeds which will result in less wear and tear on the 3.21 equipped truck.

 

[Diamondback]

Here is my example for those of you wanting a more day to day operation : this is NOT a RAM truck!

99 Cobra stock 3.27 gears .. I can launch at 5,000 RPM from a dead stop and take 2nd all the way up to 92.5 MPH (this is PERFECT for the 1/8th mile strip) .. it takes a long time (relatively) to get up to redline.
I installed 3.73s and my buddy installed 4.11s
I now can take off in 2nd at around 2500 RPM but RedLine hits very quickly.
His 4.11s take him off the line like no other and he has to hold back to keep the grip to the pavement or else he would spin tires all day and be shifting the entire time.

The gears help you get up to horsepower/torque ranges more quickly. So if you have a heavy load behind you, as stated previously the truck's drivetrain works "less" or "easier" .. go grab your kid's 10-speed. Put it in 1st or 2nd gear and take off from a dead stop .. NOW .. put it in 5th or 6th and do the same .. harder to take off in 5th/6th? What you feel is something like this : 1st/2nd = 3.92 | 5th/6th = 3.21

Not a TRUE statement of course, but a general example .. there are sooooo many other things when dealing with a vehicle drivetrain (transmission gear ratios, etc.etc.etc.)

In my Durango R/T when I drive in normal mode, it is like I am in 3.21 mode .. when I engage the SPORT mode, it is like I am now in 3.92 mode .. the transfer case also moves stuff around from say a 50:50 ratio (front : rear) to 60:40 .. the SRT allows even more selections.

Hope that gives just a little bit of insight -- I know a lot of gear heads will be like - that isn't how it works .. but I just wanted to convey how the daily driver may understand what he/she feels while sitting in the seat.

Tim N