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

[Virgiestream]

Just came across this thread. Excellent job by the OP! I have had both the 3.21 and the 3.92 and all I can say is that I absolutely love how lively the 3.92s feel with or without a trailer. the 3.21s lugged the trailer around pretty good but the 3.92s whip it it around effortlessly especially upon take off and at high speed. Yup, this vehicle sure does use more gas. About 2 miles per gallon more on average but the driving satisfaction for me has been worth the trade off. I certainly don't fault anyone for making a decision to achieve better fuel economy especially if they do not haul a trailer. So there is my very technical, seat of the pants opinion. LOL. Thanks again OP for such a thoughtful and thorough post on this topic.

Hello,

Just curious - with your build, did you stay above 1,000 lbs for your payload,
I just ordered a Limited ecodiesel with some similar options. Hoping my payload is North of 1200 lbs, I do not have the BUG, protective or sunroof

Thank you

 

[MF2020]

Hello,

Just curious - with your build, did you stay above 1,000 lbs for your payload,
I just ordered a Limited ecodiesel with some similar options. Hoping my payload is North of 1200 lbs, I do not have the BUG, protective or sunroof

Thank you

945lbs. So... not a big number but enough for my motorcycle trailer full of Harleys and 4 passengers

 

[JF19Longhorn]

945lbs. So... not a big number but enough for my motorcycle trailer full of Harleys and 4 passengers

Might want to recheck your numbers. 4 HD's has to be around 3000lbs plus the trailer.. 10-15% of that weight as tongue weight would put you at 500+/- lbs. 4 guys, even @ 150lbs puts you at 1100lbs 'cargo' before you add any gear, luggage, etc.

***misread that: I'm guessing you mean two harleys and 4 passengers. My bad!

 

[MF2020]

Might want to recheck your numbers. 4 HD's has to be around 3000lbs plus the trailer.. 10-15% of that weight as tongue weight would put you at 500+/- lbs. 4 guys, even @ 150lbs puts you at 1100lbs 'cargo' before you add any gear, luggage, etc.

***misread that: I'm guessing you mean two harleys and 4 passengers. My bad!

Correct!! We are almost maxed out with 4 relatively light passengers. Putting all our travel gear and suitcases in the trailer pushes that weight to the tongue weight which helps keep the payload down. However, I've been looking for a toy hauler and that will probably require a 2500 if I load it up with everything like this one.

 

[Bcryan]

Have a 3:21 for the mileage... pedal max to get moving quick. My friend has the 3.92 and that pedal makes has me pulling away from him. Nest of both worlds when it comes to speed

 

[Neurobit]

Have a 3:21 for the mileage... pedal max to get moving quick. My friend has the 3.92 and that pedal makes has me pulling away from him. Nest of both worlds when it comes to speed

Same here.

With my 3.21 & Pedal Commander, I have to dial it down or else I burn rubber at every take off. More than quick enough in my opinion.

 

[MF2020]

Same here.

With my 3.21 & Pedal Commander, I have to dial it down or else I burn rubber at every take off. More than quick enough in my opinion.

3.21, 3.92, Pedal Commander is just plain awesome in either gear ratio and adds so much fun to the standard driving experience! Congrats on your Pedal Commander, I absolutely love mine.

 

[Timeless]

Have a 3:21 for the mileage... pedal max to get moving quick. My friend has the 3.92 and that pedal makes has me pulling away from him. Nest of both worlds when it comes to speed

Same here.

With my 3.21 & Pedal Commander, I have to dial it down or else I burn rubber at every take off. More than quick enough in my opinion.

3.21, 3.92, Pedal Commander is just plain awesome in either gear ratio and adds so much fun to the standard driving experience! Congrats on your Pedal Commander, I absolutely love mine.

So I'm confused about these posts. When racing someone you go WOT...which my understanding of the pedal commander is that it simply applies a higher throttle input relative to the actual input.

So two trucks going WOT at the same time should not benefit from this device?

 

[Bcryan]

So I'm confused about these posts. When racing someone you go WOT...which my understanding of the pedal commander is that it simply applies a higher throttle input relative to the actual input.

So two trucks going WOT at the same time should not benefit from this device?

I was talking about my 3.21 with the Pedal Max and my friends 3.92 stock

Don’t take your traction control off from a dead start if ya like your tires

 

[silver billet]

So I'm confused about these posts. When racing someone you go WOT...which my understanding of the pedal commander is that it simply applies a higher throttle input relative to the actual input.

So two trucks going WOT at the same time should not benefit from this device?

There was a video floating around from Sprint Booster, the very first guys to come out with this kind of thing. According to the video, it also changes the speed at which WOT happens. Normally it takes some time/lag before the engine is WOT, even when you bury your foot as fast as you can. With the SB at least, they change the speed at which WOT will be reached.

So SB is like a mechanical linkage, what your foot does, the engine does. The normal pedal is like an elastic; you stomp on it as hard as you want, but still going to be some lag while the elastic expands lazily and does its stretchy thing. (elastic analogy is mine, don't blame SB for the poor analogy )

 

[Hydroblueguy]

As someone who is still waiting to pull the trigger, this is a fun conversation to watch in which everyone argues they are right.

Someone else mentioned the 3.55 ratio. I don't know the MPH/1000 revs in any gear regardless of rear end ratio. But have to think that 3.55 might well be a great balance between the two ratios. Ram can include the 3.92 as part of the towing package along with the TBC and elephant ears.
But that might just create an additional argument.....

The 3.55 are hard to find!

 

[RAM5]

I purchased a 2019 Ram Tradesman last August with the Customer Preferred Pkg 25A and the 3.92 and 5.7

I have a blast! When I drive my truck. I don’t really concern myself that it uses a bit more fuel than my car does...I’m just an old gear head at heart and this truck gives me that power, sound, shifting and that burning rubber smell I have always loved.

pick what engine and gear ratio you want, need, thinking about getting, purchase the truck and then go have fun driving such a remarkably engineered, well built, Beast of a truck!

cheers!

 

[MF2020]

I purchased a 2019 Ram Tradesman last August with the Customer Preferred Pkg 25A and the 3.92 and 5.7

I have a blast! When I drive my truck. I don’t really concern myself that it uses a bit more fuel than my car does...I’m just an old gear head at heart and this truck gives me that power, sound, shifting and that burning rubber smell I have always loved.

pick what engine and gear ratio you want, need, thinking about getting, purchase the truck and then go have fun driving such a remarkably engineered, well built, Beast of a truck!

cheers!

Now that's a GREAT review!! Love it! Could not agree more. These trucks are amazing in all different configurations. Find the one that " floats your boat" and drive it till the wheels come off! lol lol

 

[Deezl]

Thanks for all the data! As a fellow engineer I had to put it in a table.. too much text for my eyes to focus on haha.

Good statements overall, all around the 3.92 has higher ratios but in some gears, near-as-makes-no-difference for the seat-dyno.

View attachment 44185

As a fellow (Test and Integration) Sr Engineer, I thank and appreciate you for compiling the datum into a table. But you need to correct the 3.21 6th gear final drive ratio from 3.29 to 3.21. Just giving you a hard time, tongue in cheek. Stay safe and healthy!

Sent from my SM-T867U using Tapatalk

 

[TruckDriver]

As a fellow (Test and Integration) Sr Engineer, I thank and appreciate you for compiling the datum into a table. But you need to correct the 3.21 6th gear final drive ratio from 3.29 to 3.21. Just giving you a hard time, tongue in cheek. Stay safe and healthy!

Sent from my SM-T867U using Tapatalk

Corrected!

Man... HOW did I miss that?

 

[Calsun]

This comes up time and again. Put the 3.21 in 7th and it's exactly the same final gear ratio as the 3.92 in 8th. So just lock it out of 8th if it bothers you that much.

6th in the 3.21 is literally identical to 7th in the 3.92.

Once it sinks in that the top 2/3's of the gears are either very close or exactly the same as the gears in the 3.92, just 1 gear behind, you will realize that the downshifting behaviour you attribute to being a 3.21 problem would ocurr in the 3.92 as well. This is called "confirmation bias". You expect the 3.21 to be far worse, so the second it downshifts (not including 8th) then you blame it on what you expect it to be (gear ratio) rather than what is is (not enough torque; the 3.92 would have to downshift too).

The 3.92 just gets you out of the hole quicker. That's it. Once the trucks are in third or beyond, they are pretty much a direct match for eachother until the 3.92 runs out of gears and the 3.21 has one upshift left. They're just offset by 1 gear.

I mean look how close those numbers are below. You're trying to tell me you notice a difference of 0.12 in final ratio?? Not likely. Even more so for 0.0044.

View attachment 57024

Someonne has a problem with basic math as the difference in 7th gear is 0.5964 and not 0.0828. In 7th gear the 3.92 gears are 22% lower than the 3.21 gears. That translates into 25% higher engine rpms and the motor is more likely to be inside the peak power band. In 8th gear the 3.92 are 18% lower and rpms will be 21% higher. With the gas V-6 the peak torque starts is at 4000 rpm and so lower gears are much more imporant than with a diesel engine.

It also means more power when rock crawling without needing higher rpms and loosing traction on soft ground and less need to go into 4WD Low on trails.

 

[Bcryan]

4 + tree = squash .. I got it

 

[silver billet]

Someonne has a problem with basic math as the difference in 7th gear is 0.5964 and not 0.0828. In 7th gear the 3.92 gears are 22% lower than the 3.21 gears. That translates into 25% higher engine rpms and the motor is more likely to be inside the peak power band. In 8th gear the 3.92 are 18% lower and rpms will be 21% higher. With the gas V-6 the peak torque starts is at 4000 rpm and so lower gears are much more imporant than with a diesel engine.

It also means more power when rock crawling without needing higher rpms and loosing traction on soft ground and less need to go into 4WD Low on trails.

I think you're comparing 7 vs 7. Read what I wrote again:
"Put the 3.21 in 7th and it's exactly the same final gear ratio as the 3.92 in 8th".

Which it is. The number of 0.08 is the difference between 3.21 in 6 vs 3.92 in 7. Which means, more than likely, that at the same speed on the highway where the 3.21 is in 6th, the rpm will be equivalent to a 3.92 in 7. Same speed, same rpm, just different gear.

The same holds pretty much true across the last 5 gears, very minor differences.

The major differences are in first/second, and when the 3.92 has no more gears left but the 3.21 can then upshift to 8. If you're rock crawling a lot, by all means get the 3.92.

The point of my posts on all this is that the 3.92 is far less effective than some people think, because the 8 speeds give you so many gear options that you will be able to find overlapping gear ratios for the same rpm, just in a different gear. The numerical gear you are in is irrelvant, what matters is gear ratio at a given rpm, and that's my point; 6th (in the 3.21) and 7th (in the 3.92) have the same final gear ratio, so they will be at the same rpm at the same speed, giving you equal amounts of power at that speed, it doesn't matter that the one truck is in 6th and the other is in 7th.

 

[WXman]

Thanks for all the data! As a fellow engineer I had to put it in a table.. too much text for my eyes to focus on haha.

Good statements overall, all around the 3.92 has higher ratios but in some gears, near-as-makes-no-difference for the seat-dyno.

View attachment 61167

I posted a chart like this in one of these threads. There are so many of these that it gets lost. But, as you can see 6th with 3.21 is the same as 7th with 3.92. Otherwise, there are NO places where they overlap. None. No where.

Since there is zero overlapping, it's obvious that FCA is offering two different axle ratios for a reason. And, it's also obvious that the 3.92 is going to accelerate/pull harder in every single trans gear, which is why that one is most desirable and that one is the $95 option.

 

[silver billet]

I posted a chart like this in one of these threads. There are so many of these that it gets lost. But, as you can see 6th with 3.21 is the same as 7th with 3.92. Otherwise, there are NO places where they overlap. None. No where.

Since there is zero overlapping, it's obvious that FCA is offering two different axle ratios for a reason. And, it's also obvious that the 3.92 is going to accelerate/pull harder in every single trans gear, which is why that one is most desirable and that one is the $95 option.

Look, forget the word "overlap". Use "approximately equal" in chart below:

3.21 3.92
3rd ~= 4th (with a difference of 0.127)
5th ~= 6th (with a difference of 0.2851)
6th ~= 7th (with a difference of 0.0044)
7th ~= 8th (with a difference of 0.1234)

What this means, in these gears, the 3.21 will be at the same rpm, with the same final gear ratio, at the same speed (just separated by 1 numerical gear) as the 3.92. The difference noted would be what, less than 30 rpm difference in each case? This is not an opinion you can deny, sorry, it's just math and numbers.

Nobody is denying that the 3.92 will pull harder than the 3.21 when both are in 7th gear. But what you're missing, is that at the speed that the 3.92 is running at in 7th (say 50 mph), the 3.21 will not be in 7th; it will be in 6th, at that exact same speed, and same rpm, with approximately equal final gear ratio that the 3.92 has. We know that because of the final gear ratios (which determine how fast the wheel is turning on the road). If the wheels are turning at same speed in both trucks, and you can find a pair of gears such that the final gear ratio is the same (the 4 pairs listed above), then mathematically they are both running the same RPM and putting down the same power/torque (engines are identical) at the same speed. Therefore, it's not pulling any harder at a given speed (once the 3.21 has gotten off the line and shifted a few gears).

And you have no idea whatsoever on why the gear is a $95 option.