3.21 vs 3.92 Review

[Kotta390]

You burn the tires all the way through 2nd ? I guess its time for me to get the tuner for my 15 rcsb with 392 !

Would have probably been third of the TC system didn't shut down the truck lol.

 

[mhb1638]

So I changed my 3.21 open diff to a 3.92 LSD.

First impressions:

Driving like a grandma, the difference between the gears is completely moot. I cannot tell the difference other than the truck shifts earlier and more often. Driving normally has no impact IMO on how the gears differ. When you are in a more spirited driving habit, (not WOT) you can tell the difference almost immediately. The acceleration is a bit better and the truck feels "lighter" on its feet. Kind of like you lost about 1000lbs of the total curb weight. Again the shifting is much earlier and actually quicker FWIW. Highway driving is nice in that there is no more downshifting to 7th gear going up steeper hills. The truck is easier to push 8th gear and has no issue doing so.


Fuel Mileage and WOT:

Fuel mileage after a few days has dropped an average of 1-1.5mpg on the EVIC at 70mph consistent. I drive 37 miles to work and 37 miles back home and I would venture to say that is enough time on the highway to come up with this conclusion that it DID in fact drop my fuel mileage. I expected as such. Time may tell if the fuel mileage will increase slightly since the PCM is also looking for higher engine load values when it had the 3.21s.
WOT is without a doubt a nightmare for traction!!! My 3.21 open diff would blow the right tire to obliteration and shift into second, hook and go... Now with my 3.92 LSD from a dead stop the truck obliterates BOTH tires into first and all of second if not most of second! (Depending on road) I was hoping with LSD it would semi hook... I was wrong! Mind you I do have a custom tune and the low end torque has been substantially increased even with the lightest touch of the throttle. So being that I am not stock I can see maybe why the stock goodyears are having a tough time with the new setup. If I nail it in second gear, it will blow the tires off again until I go to 3rd. Overall the WOT acceleration is better. It is shifting much more now but pulling better in each gear.


Overall thoughts:

I am pretty happy with the outcome of the gears. I do recommend them to anyone who likes to have fun with their truck. The acceleration and how "light" the truck feels is refreshing. I would not however knock the 3.21s and while they are a very tall gear with a 33" tire, the 8 spd more than makes up for the lack of gearing. If you do a lot of highway driving I would recommend a 3.55 gear set. I am positive you will get the best of both worlds for the most part. I do need to invest in some better tires. The stock 20" Goodyear SRAs are absolute junk for traction with this combo. I will take the truck to the track hopefully by the end of the month and see how she does. I will keep you guys informed.

I'll add my 2 cents here...I have owned (sadly I know) about 5-6 4th gen rams. Like I said...sadly. At any rate, my current truck is my first truck with the 3.92 gears as my dealer always stocks 3.21s (this one was a trade from another dealership-a swap). I don't know if being 4WD has anything to do with it, but I can tell you for certain I DO NOT like these gears. Maybe its my MF exhaust that amplifies the transmission, IDK. But I swear the truck sounds like my dad's Chevy HD with allison transmission--always in a pulling gear. I mean that sgreat if you want to pull a house or race I guess, but as a DD, to me this is so overkill. The truck absolutely loves low gears. When on the highway if I barely push down on the throttle, 9/10 it drops several gears...I mean seriously way overkill to merge or some ****. Mileage...it dropped. In my previous 3.21 trucks (all 2wd and CC) I netted 15-16 city and 21-23 HWy. This one....13-14 city; 18-19 hwy. Again this is my impressions using over several trucks I've owned. Maybe its due to be 4WD and 3.92 gears IDK, one thing for certain, I don't like it.
As for acceleration, yea, it pulls like a beat off the line...but I think it falls little flat on the hwy...where the 3.21 gears would naturally shine. But, around town, I don't like the feeling of the tranny. Always low, always in a pull state.
Again this is just my side and experience.

 

[Custom_Ram]

I need 3.92s in my life. I can almost feel them.

But I'm buying a house this week, and getting married next month, so funds are locked down tighter than turtle *****. Nice to see your review, though, was curious about hwy MPG, I too drive a long way to work and don't want to take a huge hit.

FWIW, ditch the good years, get some 285/50/20 nexen roadian HPs. My truck has the 3.21 with factory Lsd, once the HF tune went in, it would obliterate the tires in 1st and part of second on the street. Changed to the nexen tires, minimal wheelspin on dry roads, maybe for just a sec in first. I've put 25,000 miles on them and still have half tread, no complaints.[/QUOTE

I'm surprised to see your input on your nexen tires. I have the same ones in a 305/45/22 and don't really much care for them. Traction is kinda a issue, for that reason I'm looking into getting the nitto 420s. The only reason I haven't made the switch is b.c I don't think it's gonna matter to much while trying to launch on the street. Reg cab short bed with 410s is just a tire killer, especially after I had my 3000 stall once spring hits

 

[Kotta390]

I'll add my 2 cents here...I have owned (sadly I know) about 5-6 4th gen rams. Like I said...sadly. At any rate, my current truck is my first truck with the 3.92 gears as my dealer always stocks 3.21s (this one was a trade from another dealership-a swap). I don't know if being 4WD has anything to do with it, but I can tell you for certain I DO NOT like these gears. Maybe its my MF exhaust that amplifies the transmission, IDK. But I swear the truck sounds like my dad's Chevy HD with allison transmission--always in a pulling gear. I mean that sgreat if you want to pull a house or race I guess, but as a DD, to me this is so overkill. The truck absolutely loves low gears. When on the highway if I barely push down on the throttle, 9/10 it drops several gears...I mean seriously way overkill to merge or some ****. Mileage...it dropped. In my previous 3.21 trucks (all 2wd and CC) I netted 15-16 city and 21-23 HWy. This one....13-14 city; 18-19 hwy. Again this is my impressions using over several trucks I've owned. Maybe its due to be 4WD and 3.92 gears IDK, one thing for certain, I don't like it.
As for acceleration, yea, it pulls like a beat off the line...but I think it falls little flat on the hwy...where the 3.21 gears would naturally shine. But, around town, I don't like the feeling of the tranny. Always low, always in a pull state.
Again this is just my side and experience.

I am not seeing the same issue as you are having. The truck does drives just like it did (for the most part) as it did when I had 3.21s. I am noticing the MPG drop to be closer to 1.5mpg on average now, but still not horrible. My in city fuel mileage is actually about the same with both gears and the highway pulling you speak of is strange. If I push the gas pedal down a bit, it will just pull 8th gear no issues. I do know what you mean is that the truck is sensitive with downshifting to a lower gear while passing someone, but it did the same thing if not worse with 3.21s. You either need to have your TCM adaptives reset or learn to drive the truck differently? The gears should have nothing to do with the was the exhaust sounds btw. Mine hasn't changed one bit.

 

[Kotta390]

I need 3.92s in my life. I can almost feel them.

But I'm buying a house this week, and getting married next month, so funds are locked down tighter than turtle *****. Nice to see your review, though, was curious about hwy MPG, I too drive a long way to work and don't want to take a huge hit.

FWIW, ditch the good years, get some 285/50/20 nexen roadian HPs. My truck has the 3.21 with factory Lsd, once the HF tune went in, it would obliterate the tires in 1st and part of second on the street. Changed to the nexen tires, minimal wheelspin on dry roads, maybe for just a sec in first. I've put 25,000 miles on them and still have half tread, no complaints.[/QUOTE

I'm surprised to see your input on your nexen tires. I have the same ones in a 305/45/22 and don't really much care for them. Traction is kinda a issue, for that reason I'm looking into getting the nitto 420s. The only reason I haven't made the switch is b.c I don't think it's gonna matter to much while trying to launch on the street. Reg cab short bed with 410s is just a tire killer, especially after I had my 3000 stall once spring hits

Because the gears are so low now, I don't want to go to a shorter tire like a 285/50/20. I might go 32" at the least but thats it, especially with all the highway driving I do. I will look into the Nexens, I have also been looking into the General Grabber UHP tires for more promising traction and a good price as well.

 

[mhb1638]

I am not seeing the same issue as you are having. The truck does drives just like it did (for the most part) as it did when I had 3.21s. I am noticing the MPG drop to be closer to 1.5mpg on average now, but still not horrible. My in city fuel mileage is actually about the same with both gears and the highway pulling you speak of is strange. If I push the gas pedal down a bit, it will just pull 8th gear no issues. I do know what you mean is that the truck is sensitive with downshifting to a lower gear while passing someone, but it did the same thing if not worse with 3.21s. You either need to have your TCM adaptives reset or learn to drive the truck differently? The gears should have nothing to do with the was the exhaust sounds btw. Mine hasn't changed one bit.

Well it could be many other things as well...4x4, 3k miles, etc. I'm just noticing how different this truck acts versus my previous ones. First and second seem like a 18 wheeler...like first is almost out of gear soon as you start-meaning it whines up quickly. Second is little better but still. I used the similiarity like my dad's HD. And for the exhaust, it doesn't change a thing, only amplifys what I'm seeing and feeling in the truck. I do wish the mileage wasn't so different but its there for sure. Oh and the highway thing...I may have overexaggerated that...lets say 6/10 times. Very sensitive as you say.

 

[RHeller]

The 3.92's are a blast to drive! Hell, downshift on the highway and it flat out wants to haul ass.

 

[craigsez]

tuner

WOW... What a smoking deal!! Another member on here said he could get me the Morpar gears with install kit for $350 which is a good deal.... I called around to several dealer ships and they were crazy expensive (Highest quote I got was $2200 for labor and parts).... I have a good friend who owns a shop and he quoted me $800 just for labor if I bring my own parts.... ($1150 total investment).... So.... I've been dragging my fee quite a bit..... Sure wish I was closer.... That's such a great deal!

I do have a question about your truck after you did the swap.... How did you adjust the speed O to compensate for the difference in gears? I was wondering if I wold have to take it to the dealer to make a programming change.... I have a Diablo Sport tuner but there is nothing in there that I have found that could edit this....



The trinity has a way to adjust tire size to allow for gearing differences and speedo differences as tires wear...However there is NOT any adjustability for gearing changes specifically

 

[THETANK]

WOW... What a smoking deal!! Another member on here said he could get me the Morpar gears with install kit for $350 which is a good deal.... I called around to several dealer ships and they were crazy expensive (Highest quote I got was $2200 for labor and parts).... I have a good friend who owns a shop and he quoted me $800 just for labor if I bring my own parts.... ($1150 total investment).... So.... I've been dragging my fee quite a bit..... Sure wish I was closer.... That's such a great deal!

I do have a question about your truck after you did the swap.... How did you adjust the speed O to compensate for the difference in gears? I was wondering if I wold have to take it to the dealer to make a programming change.... I have a Diablo Sport tuner but there is nothing in there that I have found that could edit this....



The trinity has a way to adjust tire size to allow for gearing differences and speedo differences as tires wear...However there is NOT any adjustability for gearing changes specifically

gears do not effect the speedo

 

[BigSloth]

Because the gears are so low now, I don't want to go to a shorter tire like a 285/50/20. I might go 32" at the least but thats it, especially with all the highway driving I do. I will look into the Nexens, I have also been looking into the General Grabber UHP tires for more promising traction and a good price as well.

True. I'll be regearing as soon I can, and I guess I'll see if I need to step up tire size.

 

[H8Speed]

I'm running 305/50/20 nitto 420's (32" tall) and I can make them hook on the street with 3.55's,we'll see what happens with 3.92's.

 

[Kotta390]

Let us know how it goes! Do you have any pictures of the rear tire setup?

 

[shortman]

Just did the swap too

Just wanted to throw in my 2 cents on this subject for others who might be considering this change.....

A little back ground about me before I give my review. I mostly drive my truck around my little town with the occasional highway drive to surrounding towns. I tow a 3 rail bike trailer with 2 or 3 bikes on it quite a bit so this was the reason for the change to be honest.... As of this post I have yest to pull my trailer but look forward to it....

With that said, I completely agree with everything the OP stated at the beginning.... Truck feels much lighter on it's feet than before.... I now don't need to always hold the truck in 6th gear to keep it from trying to drive around town at 1000 RPM in 8th gear.... I can leave in drive and it works perfectly.... No need to manually shift at all 99% of the time..... But, just as stated by the OP, the down side is the highway.... Truck looks to be turning 2K at 75 MPH where it was previously at 1600 so I'm sure there will be a fuel mileage penalty for this....

So was it worth the cost to change it? For me the jury is still out... I wanna put some more time on the truck before really making up my mind....