3.21. vs 3.92 gear

[Ramit392]

Large. Eh. Maybe "noticeable" woulda been a better way to say it.

Some others have noticed earlier that the drivetrain ratios for someone with a 3.21 in 7th gear turns out to be very close in engine RPM's to a 3.92 in 8th gear. Granted, losses between 7th and 8th may not be identical, and the engine RPM match is not perfect, but it still seems this is a relatively good way to directly compare the difference in economy.

So... I tested this... in my Hemi Durango. I wish someone with 3.21's in a Hemi Ram would also.

My Durango has the prior gen 8-speed (8HP70) but the gearing is still the same. Rear axle is 3.09. Hemi engine, obviously non-etorque. I chose a stretch of flat highway about 15 miles long with no traffic. I ran it two times back to back, both at 65 mph with the cruise set, once in 8th gear and the other in 7th. 7.5 miles out and 7.5 miles back; identical loops both times. My result?

8th gear = 26 mpg indicated
7th gear = 23 mpg indicated

My indicated is typically a couple mpg's higher than hand calculated, but regardless, a noticeable delta exists... 13%. Some might call that large.

In my humble opinion there's a lot of mis-comparisons and unqualified statements thrown around... leading people to extend discussions and draw conclusions based on things that may not warrant doing so. And we all know what happens when we hear something often enough; fiction becomes truth.

Very good comparison indeed.What I found with 3.92 was "Pushback" from the 3.92 on a 8600 lbs camper was much better to control in terms of braking,wind, inclines, wet moss covered boar ramps with out spinning, not so much with initial take off. Also in a very short distances with a need to pass at lower speeds on a two lane (not Towing any thing) it can pass very quickly and recover.To me the 3.92 is worth the 1 to 2 mpg gallon or so. But sure never had any complaints with the 3.21 some years back either.

 

[Todd]

Is the 3.92 better for towing? Of course. But is the 3.21 bad at towing? Not at all. It is pretty underrated in fact.

If you compare to a 2011-2015 F150 5.0 Max Tow w/3.73 rear end (highest offered), the 3.21 w/ZF8 has a mechanical advantage in 5 of the first 6 gears. Those F150's pulled great and were rated to tow 9000-10,300 lbs. This doesn't even account for the slight HP/Torque advantage the 5.7 has over the Ford 5.0.

It's true the 3.92 will pull better than the 3.21, but to say the 3.21 is horrible at towing (or you can't tow in the mountains within your weight limits) is nonsense. This truck with 3.21's is more capable than Ford's 6 Speed 5.0 Max Tow setup, which towed great.

2wd, This input and thought gives me hope for my configurations.

 

[NDanecker]

So... I tested this... in my Hemi Durango. I wish someone with 3.21's in a Hemi Ram would also.

My Durango has the prior gen 8-speed (8HP70) but the gearing is still the same. Rear axle is 3.09. Hemi engine, obviously non-etorque. I chose a stretch of flat highway about 15 miles long with no traffic. I ran it two times back to back, both at 65 mph with the cruise set, once in 8th gear and the other in 7th. 7.5 miles out and 7.5 miles back; identical loops both times. My result?

8th gear = 26 mpg indicated
7th gear = 23 mpg indicated

My indicated is typically a couple mpg's higher than hand calculated, but regardless, a noticeable delta exists... 13%. Some might call that large.

I think one needs to be careful with snapshot numbers as above. The above test and numbers show a difference under specific conditions. Under normal driving conditions (true tank-to-tank average that includes warm-ups, city, etc) the delta maybe larger or smaller so the above shouldn't be used as basis to decide which gives better mileage, or which is better when towing, or.....

Depending on what you are looking for should depend on what tests you conduct. If for bragging rights (max mileage) then sure - the above is a good test. If conducted right you could have found the sweet spot where MDS kicks in and ya get close to 30 mpg. Is that a true indication of what the truck will average, or what people should use when comparing fuel consumption between trucks? IMO, absolutely not. If you want to determine your fuel expense, which is what most people are interested in I think, a hand calc tank-to-tank average over thousands of miles is better.

For comparison I had a 2013 3500 diesel with 68rfe and 3.42 gears. Recorded my fuel consumption over thousands of miles with Fuelly (see below).



I traded in the truck for a 2014 3500 diesel with the only difference being the transmission and gears (AISIN and 4.10). Note this is the same driver, same drive to work with a heavier truck (AISIN adds about 200 lbs). Check out the mileage below.



Yes - the heavier truck with 4.10 gears got 0.5 mpg BETTER on average than the lighter truck with 3.42 gears.

Now ask me why I traded in the truck for an AISIN (much lower 1st gear ratio then 68rfe) and 4.10 towing gears. LOL!!!

I averaged 2 - 3 mpg better when towing heavy with the 4.10 than with the 3.42 gears, and the truck never needed to downshift above 60 mph. The 3.42 was constantly hunting between 5 and 6 sometimes I had to lock out 6th. Also struggled to get moving when towing my 25k GN dump trailer (gross was around 36k) where the AISIN had no trouble.

IMO - if you tow anything close to capacity with the RAM get the 3.92. You will me much happier. Can the 3.21 tow? Sure but there is a cost associated with it and sometimes it's not as much as some people make you believe.

EDIT: And yes - I was getting better mpg in a 9k lb dually then my 5k lb 1500 Ram. LMAO!

 

[Trooper4]

A little late to the party, but will throw my hat into the ring also. Just bought a Limited with the e-torque and 3.92's in December in Tucson, and just returned to Washington state a coupe of weeks ago. I am pulling a 9000lb fifth wheel, up to Vegas, Bakersfield, Portland, The Dalles, and over the pass to central Washington. Totaled about 3000mi, over hill and dale, and averaged just over 10mpg. On the flats it was up to around 12-13, but on the long hills dropped to about 3-4, but overall average was 10-11. This is calculated mpg, not truc computed, although I did notice that the truck and my figures were within a couple of tenths. Speeds were in the 50-60 range on the trip, with the occasional fast passing, but kept it at a comfortable speed. I will say that towing with a softer suspension than what I am used to took some getting used to, the truck did exceptionally well, and the softer ride is a welcome change for me as I get older and stiffer. I think that a lower rearend would work well in flats and low hills, but I would go with the higher(3.92) if you spend a lot of time in higher hills and mountain terrain. When running empty, I wish I had the lower rearend for better mileage, but can still get decent mpg if I keep the speed down. Whatever you get, enjoy. You will be getting a grate truck.

 

[1guyandtools]

I’m looking to place an order and I’m not sure of which gear ratio to order. 95% of the time I will be driving the truck empty. Gas mileage is important to me so I’m ordering the e-torque engine. I tow a small trailer that weighs 4000 lbs and only do it 6-8 weekends a year.

My concern is that the 3.92 gear is going to kill the gas mileage. The 3.21 just seems a bit low for a truck of this size.

Looking for the mpg difference between the two in every day driving.

Thanks

Go with the 3.21 You are going to pull that 4000 around and not even know its back there with an 8 speed trans

 

[mjamesgt]

A little late to the party, but will throw my hat into the ring also. Just bought a Limited with the e-torque and 3.92's in December in Tucson, and just returned to Washington state a coupe of weeks ago. I am pulling a 9000lb fifth wheel, up to Vegas, Bakersfield, Portland, The Dalles, and over the pass to central Washington. Totaled about 3000mi, over hill and dale, and averaged just over 10mpg. On the flats it was up to around 12-13, but on the long hills dropped to about 3-4, but overall average was 10-11. This is calculated mpg, not truc computed, although I did notice that the truck and my figures were within a couple of tenths. Speeds were in the 50-60 range on the trip, with the occasional fast passing, but kept it at a comfortable speed. I will say that towing with a softer suspension than what I am used to took some getting used to, the truck did exceptionally well, and the softer ride is a welcome change for me as I get older and stiffer. I think that a lower rearend would work well in flats and low hills, but I would go with the higher(3.92) if you spend a lot of time in higher hills and mountain terrain. When running empty, I wish I had the lower rearend for better mileage, but can still get decent mpg if I keep the speed down. Whatever you get, enjoy. You will be getting a grate truck.

You mentioned softer suspension than you are used to. What are you used to towing with? Do you find the air suspension too soft when towing? Whats the tongue weight on your trailer and whats your payload? I would think you are getting very close to the payload ratings if you are putting 1300 ish tongue weight on it and have and gear or people in the car with you.

 

[Joeyp]

You mentioned softer suspension than you are used to. What are you used to towing with? Do you find the air suspension too soft when towing? Whats the tongue weight on your trailer and whats your payload? I would think you are getting very close to the payload ratings if you are putting 1300 ish tongue weight on it and have and gear or people in the car with you.

Get the 3.92..... truck will be much happier like mentioned above plus if you ever want to put a larger tire on down the road.

 

[Jus Cruisin]

I'll be towing with 3.21's, enclosed car trailer. I'll be dragging 6,500 lbs +/-. I tow about a half dozen times a year but when I do, it's a distance. I have no concerns, especially with the 8 sp. With the 3.92's, I might be in 7th for instance and 3.21's might be in 6th to be at the same rpm. No big deal to me. I pull through the Carolina hills getting from Florida to Michigan and back. That's it. I've towed with diesels and an ecoboost. None were strained. I don't figure this one will be either.

 

[mjamesgt]

Get the 3.92..... truck will be much happier like mentioned above plus if you ever want to put a larger tire on down the road.

I’m referring to his soft suspension comment, not the rear end ratio...


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[Trooper4]

You mentioned softer suspension than you are used to. What are you used to towing with? Do you find the air suspension too soft when towing? Whats the tongue weight on your trailer and whats your payload? I would think you are getting very close to the payload ratings if you are putting 1300 ish tongue weight on it and have and gear or people in the car with you.

I used to pull a 16000lb fifth wheel with a HD 2500 2005 Duramax, but traded the trailer for a 1/2 ton towable, 8900lb fifth wheel last year. The towing/load capability was no longer needed, and the stiff suspension on most states rough roads just was too jarring with the smaller trailer. My Ram is rated at 1700 gvw, and 14000 towing, so I am within the truck ratings. The softer ride of the Ram is quite nice, but I was used to a more solid feel with no side movement, so you can see what a difference that would make in my case. Admittedly I am at the upper end of the rating, but still well within it

 

[mjamesgt]

I used to pull a 16000lb fifth wheel with a HD 2500 2005 Duramax, but traded the trailer for a 1/2 ton towable, 8900lb fifth wheel last year. The towing/load capability was no longer needed, and the stiff suspension on most states rough roads just was too jarring with the smaller trailer. My Ram is rated at 1700 gvw, and 14000 towing, so I am within the truck ratings. The softer ride of the Ram is quite nice, but I was used to a more solid feel with no side movement, so you can see what a difference that would make in my case. Admittedly I am at the upper end of the rating, but still well within it

Thanks, you had me worried the air suspension was too soft.


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[SColang22]

Has anyone see anything on changing gears? I put some bigger tires on and want to go from 3.21 to 3.92. Anyone know what to do with Rams “sealed” front axle. With a 4x4 I need to change both but can’t find anything on changing the front end without doing a whole new axle

 

[Joeyp]

Has anyone see anything on changing gears? I put some bigger tires on and want to go from 3.21 to 3.92. Anyone know what to do with Rams “sealed” front axle. With a 4x4 I need to change both but can’t find anything on changing the front end without doing a whole new axle

Supposedly the dealers want nothing to do with it. They told me that it was 8 to 10 hours of labor per axle. I ended up canceling the truck that I ordered and re-ordered a new one with the 3.92

 

[SColang22]

Supposedly the dealers want nothing to do with it. They told me that it was 8 to 10 hours of labor per axle. I ended up canceling the truck that I ordered and re-ordered a new one with the 3.92

I’m doing it after market. The problem is the shop says ram “sealed” the front end axle so I don’t know what I need to do there. I have the new gears for the rear already

 

[Joeyp]

I’m doing it after market. The problem is the shop says ram “sealed” the front end axle so I don’t know what I need to do there. I have the new gears for the rear already

Gotcha.... hope you get everything sorted out. Post something when you’re done so we know how you made out

 

[riccnick]

Supposedly the dealers want nothing to do with it. They told me that it was 8 to 10 hours of labor per axle. I ended up canceling the truck that I ordered and re-ordered a new one with the 3.92

Even they should be ashamed of that quote!

 

[riccnick]

I’m doing it after market. The problem is the shop says ram “sealed” the front end axle so I don’t know what I need to do there. I have the new gears for the rear already

It's nothing more than "sealed" with RTV. "Sealed" has come to mean it isn't supposed to need service, it doesn't actually mean its air or water tight, or can't be taken apart. Most likely the front diff has a drive axle housing that extends across to the passenger side for the equal length CV axles to connect to, which would need to be removed in order to remove and access the front diff.

 

[SColang22]

It's nothing more than "sealed" with RTV. "Sealed" has come to mean it isn't supposed to need service, it doesn't actually mean its air or water tight, or can't be taken apart. Most likely the front diff has a drive axle housing that extends across to the passenger side for the equal length CV axles to connect to, which would need to be removed in order to remove and access the front diff.

That’s what I figured but apparently ram hasn’t put out much info on them. Gonna try another shop this week.

 

[riccnick]

No idea who makes the axles for ram, but I'd guess its a Dana front section and it's probably a revision of an existing design, so anyone who's worked on a half ton truck in the last 30 years should be able to take one apart. There's nothing revolutionary about the DT Ram's axles and drivetrain. It's all purchased from suppliers like Dana and AAM, just like everyone else. Any competent off-road shop would be able to do the job for you in a few hours, whoever is making a big deal out of this is either, A: trying to price gouge you, or B: incompetent.

 

[SColang22]

No idea who makes the axles for ram, but I'd guess its a Dana front section and it's probably a revision of an existing design, so anyone who's worked on a half ton truck in the last 30 years should be able to take one apart. There's nothing revolutionary about the DT Ram's axles and drivetrain. It's all purchased from suppliers like Dana and AAM, just like everyone else. Any competent off-road shop would be able to do the job for you in a few hours, whoever is making a big deal out of this is either, A: trying to price gouge you, or B: incompetent.

So i found someone who can do it but the problem is the front diff has welds which makes if very difficult to change the gear and pinion. The cheapest option is to replace the hole front end housing with the 3.92 but that from the factory only currently and cost 1400. So in total its 2500-3000 to change it over without doing some janky stuff. I'm only wanting it to run 35s and do some lighter towing so

No idea who makes the axles for ram, but I'd guess its a Dana front section and it's probably a revision of an existing design, so anyone who's worked on a half ton truck in the last 30 years should be able to take one apart. There's nothing revolutionary about the DT Ram's axles and drivetrain. It's all purchased from suppliers like Dana and AAM, just like everyone else. Any competent off-road shop would be able to do the job for you in a few hours, whoever is making a big deal out of this is either, A: trying to price gouge you, or B: incompetent.

so I found someone who can do it but the problem is the front diff has some welds which makes it almost impossible to change the gear and pinion. The best option is to buy a new housing from ram but they are running $1400. So in total for both axles I’m looking 2500-3000 installed without doing some janky stuff to the front end. I’m only wanting to change because I’m running 35’s and still want to tow some light stuff but for that much I’ll wait to see if it gets cheaper or I’ll just keep what I got