3.92 with the Ecodiesel

[ammdrew]

Also I believe you have some mixed mileage claims speed is the largest killer of mileage as they are still bricks at 80mph.. that’s pushing a lot of air and side wind can even hurt mileage worse then straight on... winter fuel hurts mileage also so all of the states that deal with freezing temps will have 2-4 mpg loss for 4-6 months a year.

 

[Dirtdoc]

So I guess I am an idiot for having 3.92's in my eco and getting 29+ mpg on the highway? Wow

 

[Willwork4truck]

Suit yourself You can lead a horse to water but can't make him drink. As I said several posts ago, I'm done with this discussion, all the math and numbers are in the "engineers guide" thread.

Agree, too much arguing... the main advantage of 3.92 is starting off and if running taller or bigger tires. Burns gas faster too.
Resale is likely higher cause’ peeps think they “need” it.

I’d have bought 3.92’s if the truck was the right colors (wifes). As it was I got 3.21.

Life is too short to stress over gears, unless that’s a persons “life” (trolls for example and no I’m not pointing fingers).

I needed 4.10’s “back in the day” with my 1977 200 hp GMC 400 small block CC 1 ton 4x 3 speed auto... It couldnt get out its own way otherwise, even without the 28’ woodframed TT behind it.

Todays trucks/drivetrain, brakes are, uhh, different..

 

[Willwork4truck]

As someone deciding between 3.21 and 3.92, this back and forth makes my head spin. I'm probably mid-level when it comes to car knowledge. It seems like the transmission shifts based on RPMs and not MPH, and therefore it's taking advantage of the ratio in a particular gear? Transmissions are smart enough to choose the right ratio for the situation, correct? Whether that means my hypothetical 3.21 is in 5th or my 3.92 is in 6th?

My main curiosity is why the 3.92 has a higher overall tow rating. Is it because the 3.21 would be strained to get off the line with 9K+ being towed?

I do agree with those who wish there was a 3:55 option. I have zero interest in larger tires, but will be towing a 6000# TT with some frequency. The truck won't be a daily driver, but will be used for work (hauling stuff) and long trips (with and without the TT) so I'd really like to find an in-depth look at the MPG difference.

I’d say thats a good reason for a mid range gear choice, like Ferd offers...

 

[Willwork4truck]

3.92 for you, don’t over think this. If you’re towing a 6k traveler trailer frequently it’s a no brainer

Key word is “frequently”.
Now for a thread arguing about how many times is “frequent”.

 

[nc_beagle]

Key word is “frequently”.
Now for a thread arguing about how many times is “frequent”.

Yep. Frequently is a tough thing to judge. I hope we get to make a lot of trips. Though, in addition to the TT, I could end up towing some heavy trailers as needed. I'm not the contractor, but I'm involved in a small construction/manufacturing company -- the size where everybody does a little of everything.

 

[Willwork4truck]

And you have a truck which makes you “popular”. A covered 6x10 or 12’ would make you even more popular, on Saturdays near mid or end of the month...

 

[Minicoal]

Interstimg discussion however some of old wisdom related to higher ratio does get lost with more modern 8 and 10 speed transmissions of today. ...and then there are incredible torque numbers at low rpms in modern small displacement diesels of today. I agree with academic argument that there is no denying mechanical advantage of 3.92 has in all gears Verses 3.21

A lot of the discussion would be more relevant couple Decades ago with Ram 1500 5 speed transmissions with Hemi v8.

For most of us towing under 6000 lbs 3.21 is fine given number of gears and how smooth transmissions of today shift. Especially with torque Ecodiesel makes at low rPM. If we were talking Hemi v8 with lower torque in that lower rpm range 3.92 would make much bigger difference in my opinion. ....also if you’re towing more than 6000 lbs with a 1500 series truck I might question wisdom of using a 1500 rather than more appropriate Ram 2500 or 3500 just due to payload limitations of 1500.

the other thing many forget is that tire diameter also plays into equation. There’s a reason why rebels come with 3.92. Taller tires. Cheap mans gear ratio tweak is to use smaller diameter tires. I always chuckle when I see people talking about higher ratio being better for towing yet they sit on huge diameter off road tires . Mechanical advantage negated. ....ooops. On internet what’s one of first thing new truck owners do? Get those awesome new wheels with taller tires. So for those planning on bigger tires 3.92 is must have.

if I towed all time and lived where I towed uphill all the time I’d likely opt for 3.92 or if I wanted to beat a 3.21 Ecodiesel in stoplight race or spent lots of time off-roading with big tires there would be real advantages Of 3.92. For typical 1500 ED owner who tows reasonable weights a 3.21 is fine.

For me the truck truly is daily driver , utility vehicle, maybe once or twice month hauler for my Feather weight trailer with race car or under 6000 lb travel trailer.

I do have to laugh first day I was driving my new 21 Laramie ED in 2wd and I accidenty burned out from a stopligHt. First burnout for me in years and I have been driving AWD SRT 6.4 Hemi Durango for couple years and never could never burnout .

‘I’m curious are there any youtube drag races between 3.92 and 3.21 ED of similar trims and tire sizes? Think it would be very eye opening to some at how much quicker 3.92 would be.

 

[Dadeoo]

No, don't get stuck on a technicality. The difference is insignificant, which is the point.

You are so, so, wrong. The small differences are exactly the point of the 3.9 ratio -- when towing moderate to heavy loads and especially a wind resistant boxy trailer. The small differences in gear ratios are what it's all about. Why do you think FCA offers it? It increases engine speed just a bit and that is needed to get the maximum tow rating. While the 3.9 ratio doesn't deal with the limited cargo capacity issue of a half ton, it does really help a half ton 3.0 diesel engine tow a bumper pull trailer that resembles a sail, like mine.

On the other hand, if you buy ecodiesel and don't tow much, then you are right, the 3.9 is indeed completely pointless.

 

[silver billet]

You are so, so, wrong. The small differences are exactly the point of the 3.9 ratio -- when towing moderate to heavy loads and especially a wind resistant boxy trailer. The small differences in gear ratios are what it's all about. Why do you think FCA offers it? It increases engine speed just a bit and that is needed to get the maximum tow rating. While the 3.9 ratio doesn't deal with the limited cargo capacity issue of a half ton, it does really help a half ton 3.0 diesel engine tow a bumper pull trailer that resembles a sail, like mine.

On the other hand, if you buy ecodiesel and don't tow much, then you are right, the 3.9 is indeed completely pointless.

The 3.92 and the 3.21 have pretty much the same gear ratios available. The 3.92 has a lower first, the 3.21 has a taller final, the rest they share. By "share" I mean that they have gear ratios that are pretty much identical. Do you think there is a significant difference between a gear ratio that is "3.21" vs "3.2144"? That is the differences I'm referring to.

When you have enough gear ratios available such that you only need 400 rpms to upshift to the next one, a difference of "0.0044" is not even going to change your RPMs by more than, what, 10 rpms? I'm too lazy to do the math. That is the most extreme example, but the other ratios are pretty much identical as well, and none of them are going to be more than 30 to 50 rpms.

So yes, if you're going down the road in your 3.92 rear end at 2100 RPMs, and me going down the road at same speed at 2130... nobody cares, it changes nothing with respect to power or fuel efficiency. Yes there is still a tiny difference, but it is completely insignifcant to the point where other factors are much more important (tire pressure, weight in the truck, top tier gas vs crummy gas, octane rating etc etc).

 

[Rebelguy2020]

The 3.92 and the 3.21 have pretty much the same gear ratios available. The 3.92 has a lower first, the 3.21 has a taller final, the rest they share. By "share" I mean that they have gear ratios that are pretty much identical. Do you think there is a significant difference between a gear ratio that is "3.21" vs "3.2144"? That is the differences I'm referring to.

When you have enough gear ratios available such that you only need 400 rpms to upshift to the next one, a difference of "0.0044" is not even going to change your RPMs by more than, what, 10 rpms? I'm too lazy to do the math. That is the most extreme example, but the other ratios are pretty much identical as well, and none of them are going to be more than 30 to 50 rpms.

So yes, if you're going down the road in your 3.92 rear end at 2100 RPMs, and me going down the road at same speed at 2130... nobody cares, it changes nothing with respect to power or fuel efficiency. Yes there is still a tiny difference, but it is completely insignifcant to the point where other factors are much more important (tire pressure, weight in the truck, top tier gas vs crummy gas, octane rating etc etc).

Just to clarify a few things that you mentioned.
The rpm will have more of a difference when cruising in 8th gear, there will be more than 30 rpm difference, on the 3.92 the 8th gear is almost equivalent to the 7th speed on the 3.21.
The transmission is the same for both the 3.92 and the 3.21 rear axle, the 3.92 has a higher towing capabilities because of the wider ratio, meaning it takes less effort to turn the pinion gear that is driving the crown gear, less effort, less heat, more durability. The downside is that it does take more revolutions (or partial revolution) of the pinion to turn the crown gear one complete turn, so 3.92 revolutions of the pinion for 1 revolution of the crown gear compared to the other rear differential 3.21 revolutions of the pinion for 1 revolution of the crown gear.
That is why you see the more noticeable difference in the 1st gears for take off purpose, less effort from a stop because of the higher ratio, then the other noticeable difference is in the 8th gear, the 3.92 will have a higher rpm than the 3.21.
The bottom line is that it takes less effort to turn the 3.92 rear axle in all of the 8 gears of the transmission.
I hope my explanation is understandable.

 

[silver billet]

Just to clarify a few things that you mentioned.
The rpm will have more of a difference when cruising in 8th gear, there will be more than 30 rpm difference, on the 3.92 the 8th gear is almost equivalent to the 7th speed on the 3.21.
The transmission is the same for both the 3.92 and the 3.21 rear axle, the 3.92 has a higher towing capabilities because of the wider ratio, meaning it takes less effort to turn the pinion gear that is driving the crown gear, less effort, less heat, more durability. The downside is that it does take more revolutions (or partial revolution) of the pinion to turn the crown gear one complete turn, so 3.92 revolutions of the pinion for 1 revolution of the crown gear compared to the other rear differential 3.21 revolutions of the pinion for 1 revolution of the crown gear.
That is why you see the more noticeable difference in the 1st gears for take off purpose, less effort from a stop because of the higher ratio, then the other noticeable difference is in the 8th gear, the 3.92 will have a higher rpm than the 3.21.
The bottom line is that it takes less effort to turn the 3.92 rear axle in all of the 8 gears of the transmission.
I hope my explanation is understandable.

Only the first 2 gears have a difference my friend. And the very last one in the 3.21. Beyond that, both trucks will give you approximately equal gear ratios to play with, they just occur at different spots. So yes exactly, the 8th gear in the 3.92 is pretty much equal to 7th gear in 3.21. Therefore, since the gear ratios are the same, then both trucks will be using the same RPMs at the same speed. There is no difference between the trucks when the one is in 8th and the other 7th.

Your mistake is assuming both trucks will be in the same gear (8th vs 8th) at the same time. They won't. Both trucks will be in the same gear ratio.

 

[VernDiesel]

We had a guy on another forum towing a large camper at the rated capacity of his Pentastar powered 1500. Trying to tow the grade from Cali to Vegas. If memory serves me he could only hold 2 gear meaning he couldn’t make or hold the upshift to 3rd. So he screamed up it slowly and at high temps. If he had the 3.92 instead of the 3.21 not only would of he had 3 gears to work with instead of two because the jump wasn’t as many rpms & more dynamics he might have been able to make and hold 4th gear at less rpm. More gears to split what final drive ratio the motor will handle is a significant benefit in itself. Working with 4 gears or at least 3 instead of 2 at the limits is again significant in itself.

Another one that is difficult to explain is acceleration rate. So you would think after first gear the acceleration rate benefit of the 3.92 over the 3.21 would be gone but it’s not. The 3.92 truck will keep out accelerating the 3.21 truck.

 

[silver billet]

We had a guy on another forum towing a large camper at the rated capacity of his Pentastar powered 1500. Trying to tow the grade from Cali to Vegas. If memory serves me he could only hold 2 gear meaning he couldn’t make or hold the upshift to 3rd. So he screamed up it slowly and at high temps. If he had the 3.92 instead of the 3.21 not only would of he had 3 gears to work with instead of two because the jump wasn’t as many rpms & more dynamics he might have been able to make and hold 4th gear at less rpm. More gears to split what final drive ratio the motor will handle is a significant benefit in itself. Working with 4 gears or at least 3 instead of 2 at the limits is again significant in itself.

Another one that is difficult to explain is acceleration rate. So you would think after first gear the acceleration rate benefit of the 3.92 over the 3.21 would be gone but it’s not. The 3.92 truck will keep out accelerating the 3.21 truck.

Depending on which gear you're upshifting into, the difference is about 400 rpms. It's not going to change much of anything, and he definitely wouldn't be able to hold 4th. So if his truck is "screaming" with the 3.21, at best he is going to be up to 400 rpms lower in the same situation. Personally if I was in that situation a lot, an engine that is screaming in second is a pretty clear indication that he doesn't have enough truck, and it's just a matter of a slightly larger hill before he gets the same situation back in the 3.92. Just buy a bigger truck

Same thing with acceleration. That only holds true under 100% WOT, and I fail to see a case where the 3.21 under WOT is not acceptable but the 3.92 is, I think we're splitting hairs here again and is a second indication that his truck isn't big enough. No doubt the 3.92 gives you a slightly bigger buffer, I don't deny it, but you picked 2 very extreme examples which to me suggest his truck isn't big enough anyway. In your examples, I wouldn't be comfortable in either truck under those conditions.

 

[WXman]

So I guess I am an idiot for having 3.92's in my eco and getting 29+ mpg on the highway? Wow

"Idiot" is a strong word. "Unicorn" might be better, if the half-million+ combined miles of real world info from owners is to be believed.

the other thing many forget is that tire diameter also plays into equation. There’s a reason why rebels come with 3.92. Taller tires. Cheap mans gear ratio tweak is to use smaller diameter tires. I always chuckle when I see people talking about higher ratio being better for towing yet they sit on huge diameter off road tires . Mechanical advantage negated. ....ooops. On internet what’s one of first thing new truck owners do? Get those awesome new wheels with taller tires. So for those planning on bigger tires 3.92 is must have.



‘I’m curious are there any youtube drag races between 3.92 and 3.21 ED of similar trims and tire sizes? Think it would be very eye opening to some at how much quicker 3.92 would be.

Well no kidding. The Rebel has 33s, which are 3% taller. Hardly enough to justify a 22% increase in gear ratio. It has more to do with pushing around the extra weight, and providing a mechanical advantage while off road. On the topic of taller tires though, one of the main reasons guys DO buy the 3.92 gears is so that the truck will still be drivable on 35" or larger tires. A truck with 3.21s that a guy throws 35" tires on is now effectively turning the RPM of a stock truck with 2.85 axle gears. That would be pathetic. Yet another reason the 3.92s are superior.

Will the 3.92 ratio truck be faster 0-60 than the 3.21 ratio truck? That would be interesting to see. We know that in the performance world, one of the very first mods guys make to their cars is a numerically higher axle ratio.

We had a guy on another forum towing a large camper at the rated capacity of his Pentastar powered 1500. Trying to tow the grade from Cali to Vegas. If memory serves me he could only hold 2 gear meaning he couldn’t make or hold the upshift to 3rd. So he screamed up it slowly and at high temps. If he had the 3.92 instead of the 3.21 not only would of he had 3 gears to work with instead of two because the jump wasn’t as many rpms & more dynamics he might have been able to make and hold 4th gear at less rpm. More gears to split what final drive ratio the motor will handle is a significant benefit in itself. Working with 4 gears or at least 3 instead of 2 at the limits is again significant in itself.

Another one that is difficult to explain is acceleration rate. So you would think after first gear the acceleration rate benefit of the 3.92 over the 3.21 would be gone but it’s not. The 3.92 truck will keep out accelerating the 3.21 truck.

Yeah, a few weeks ago I was pulling my enclosed trailer with my new tractor that I just picked up from the dealer. Probably 8,000 lbs. total. When I get home, I have to climb a 400 foot paved hill of steep grade at less than 10 MPH that enters our property. When I hit the hill that day with the trailer in tow, the truck went down to 1st gear and still almost stalled. At one point the tach was down to 1,100 RPM and I could feel that the truck was on the verge of stalling out. Clearly I was at the limit of what this powertrain wanted. The first thought that went through my head is, "If I had the 3.21 axle gear, I literally wouldn't have made it home." In actuality, I could have dropped the t-case into low range and would have made it just fine. But the 3.92s literally were the difference between having to do that and not having to do that.

 

[Uga40]

"Idiot" is a strong word. "Unicorn" might be better, if the half-million+ combined miles of real world info from owners is to be believed.



Well no kidding. The Rebel has 33s, which are 3% taller. Hardly enough to justify a 22% increase in gear ratio. It has more to do with pushing around the extra weight, and providing a mechanical advantage while off road. On the topic of taller tires though, one of the main reasons guys DO buy the 3.92 gears is so that the truck will still be drivable on 35" or larger tires. A truck with 3.21s that a guy throws 35" tires on is now effectively turning the RPM of a stock truck with 2.85 axle gears. That would be pathetic. Yet another reason the 3.92s are superior.

Will the 3.92 ratio truck be faster 0-60 than the 3.21 ratio truck? That would be interesting to see. We know that in the performance world, one of the very first mods guys make to their cars is a numerically higher axle ratio.



Yeah, a few weeks ago I was pulling my enclosed trailer with my new tractor that I just picked up from the dealer. Probably 8,000 lbs. total. When I get home, I have to climb a 400 foot paved hill of steep grade at less than 10 MPH that enters our property. When I hit the hill that day with the trailer in tow, the truck went down to 1st gear and still almost stalled. At one point the tach was down to 1,100 RPM and I could feel that the truck was on the verge of stalling out. Clearly I was at the limit of what this powertrain wanted. The first thought that went through my head is, "If I had the 3.21 axle gear, I literally wouldn't have made it home." In actuality, I could have dropped the t-case into low range and would have made it just fine. But the 3.92s literally were the difference between having to do that and not having to do that.

I have a stock 3.92 and get 28 on hwy and 24 in the city. Truck reads a 1 mpg more than hand calculation

Sent from my SM-G965U using Tapatalk

 

[Rebelguy2020]

Only the first 2 gears have a difference my friend. And the very last one in the 3.21. Beyond that, both trucks will give you approximately equal gear ratios to play with, they just occur at different spots. So yes exactly, the 8th gear in the 3.92 is pretty much equal to 7th gear in 3.21. Therefore, since the gear ratios are the same, then both trucks will be using the same RPMs at the same speed. There is no difference between the trucks when the one is in 8th and the other 7th.

Your mistake is assuming both trucks will be in the same gear (8th vs 8th) at the same time. They won't. Both trucks will be in the same gear ratio.

We both have the same understanding, everything makes sense.
On your last comment about the transmission shifting pattern for both trucks, you are absolutely right, I didn’t explain myself clearly, what I meant is that if you often towed a heavy trailer in the city or for long hauls, the 3.21 rear end would be working harder, in all gears compared to the 3.92.

 

[silver billet]

We both have the same understanding, everything makes sense.
On your last comment about the transmission shifting pattern for both trucks, you are absolutely right, I didn’t explain myself clearly, what I meant is that if you often towed a heavy trailer in the city or for long hauls, the 3.21 rear end would be working harder, in all gears compared to the 3.92.

You mean more stress? I'll give you that, but rear end failure is not very common, in fact I've yet to hear of it on this forum or the other one. I would never spend any time worrying about that at all. Guys run super chargers on stock rear ends and transmissions and as far as I can tell none of that increased torque has any effect downstream (pistons and rings, yeah they don't like being pushed past about 5 psi).

 

[Rebelguy2020]

You mean more stress? I'll give you that, but rear end failure is not very common, in fact I've yet to hear of it on this forum or the other one. I would never spend any time worrying about that at all. Guys run super chargers on stock rear ends and transmissions and as far as I can tell none of that increased torque has any effect downstream (pistons and rings, yeah they don't like being pushed past about 5 psi).

You’re right again, I should have said more effort to turn the 3.21.
I did have a 3.92 rear end failure on my 2011 Ram, it was under a year old, I had the dealer repair the parking brake then on my way home (on the highway) I was starting to hear a weird noises and the differential locked up and both rear wheel stopped instantly, I was able to skid to the side of the highway. I saw fresh red silicone on the rear differential. Turns out that they had to open the rear end to service the parking brakes, at the beginning they had tried to tell me the pinion bearing just happened to seize, but then admitted to forget to fill the differential with oil. I had driven about a mile in town and about a mile on the highway when it seized. They gave me a rental truck while they fixed their f up.

 

[c3k]

Lots of good stuff and funny comments (intended and otherwise ) in this thread.

I went with the "diesel has a lot of torque and a 3.21 is all I need" school of thought. It was a bit of a guess. I don't tow much and what I anticipate will be mostly a 6x12 single-axle utility trailer (2' walls, 4' drop gate/ramp).

Once I get my truck, I'll run it as is (ORP) for about a year. Then I plan on a moderate lift (2" max), and 34" tires. I'm hoping that won't be too much for the diff ratio. If y'all see a nice set of 34" tires on sale, slightly used, a year from now, you'll now it didn't work.