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

[silver billet]

Well I’ll be dammed. 3.92 is just a gimmick.

It's not a gimmick. But you can't just make stuff up about it either. What you posted was wrong, and I pointed out the reason why.

 

[bwsRam19]

The same argument can be used for Hemi vs. V6, if you didn't need the power why didn't you buy the V6. I bought the biggest engine and the lowest gears(3.92), because thats what I like. At no point during the buying process did I think about gas mileage. To each their own, I guess. I just don't understand how you have more options, its pretty plain to me: 3.21 you get a little better mileage on the highway, less towing/performance grunt in any selected gear, 3.92 little less gas mileage on the highway, more towing/performance grunt in any selected gear. You talk about gearing down to match the 3.92, eventually you run out of gears.

 

[silver billet]

The same argument can be used for Hemi vs. V6, if you didn't need the power why didn't you buy the V6. I bought the biggest engine and the lowest gears(3.92), because thats what I like. At no point during the buying process did I think about gas mileage. To each their own, I guess. I just don't understand how you have more options, its pretty plain to me: 3.21 you get a little better mileage on the highway, less towing/performance grunt in any selected gear, 3.92 little less gas mileage on the highway, more towing/performance grunt in any selected gear. You talk about gearing down to match the 3.92, eventually you run out of gears.

The difference between the hemi and v6 is pretty large; approx 100 hp, and what, 130? 140? lb/feet of torque? It's noticable at every speed, in every gear, and limits your towing. The difference between the 3.21 and 3.92 is quite small, and only occurs in first + second + eight, and only while under hard acceleration scenarios (towing). So you're right that the question is very similar, the difference is in how close the 3.21 vs the 3.92 is, versus the v6 and hemi in general which is more significant.

I'm not saying the 3.21 gives you more options, just that you have the option of using gear limiting in case the occasional extra downshift bothers you.

But you still don't understand the towing issue; because you don't get more grunt with 3.92 in every gear. That's literally the whole point. You get more grunt in first, until it upshifts to second; because the 3.92 has to upshift first (before the 3.21), that means that right at that point, the 3.21 is still in first, quite close to peak hp/torque, and 3.21 in first puts out more power than 3.92 in second. So at that point the 3.21 is making more grunt, until it as well has to upshift a few seconds later.

You're still thinking that both trucks will be in the same numerical gear for the same speed. But they won't be. The 3.92 will be in 7th doing 1300 RPM's, and at the exact same speed, the 3.21 will be in 6th, not 7th. We know that because if you do the math, the gear ratio of 3.92 in 7th is identical to the gear ratio of 3.21 in 6th. So since those gear ratios are identical, and the trucks are going the same speed, then the RPM's MUST be indentical (that's how gears work). If RPM's are equal, and they're going the same speed, then they're putting down the same power at that speed.

The 3.92 makes more power in 7th, then the 3.21 does in 7th; but they won't be in 7th together at the same time/speed. When the 3.92 is in 7, the 3.21 is in 6, using an identical gear ratio.

So; only in first and second does the 3.92 make more grunt. After that, they're pretty much equal, just separated by 1 numerical gear. On the highway, the 3.21 will pull just as strongly as the 3.92, it will just use lower gears to do it. If the 3.92 can pull the load in 7th, the 3.21 will do it in 6th putting down the same RPMs and power. If the 3.92 needs to use 4th to go up a steep hill, the 3.21 will use 3rd instead, and since those gear ratios are approximately equal between the trucks, they will both be putting down the same power at that speed.

 

[bwsRam19]

 

[BowDown]

I think you are overestimating the whole "equalizing" of the gears of the trans because it was posted that there is a way that the final drive ratio would be similar between the two sets of rear gears

No, Im very well aware of how much the trans plays a role in this. My hobby is racing, the ZL1 camaro with the M6 is a dog, th ZL1 with the 10spd auto is a monster yet it has 2.85 gear. Why? The 10 speed trans, the 10 forward gears eliminates the need for a deep rear gear and keep the engine in the meat of the torque curve through the pull, the very same thing the ZF 8 speed does in these trucks.
The rear deeper gear ratio's can be replicated by a deep first gear in the trans then slowly eliminated through the use of multiple forward gears

 

[silver billet]

No, Im very well aware of how much the trans plays a role in this. My hobby is racing, the ZL1 camaro with the M6 is a dog, th ZL1 with the 10spd auto is a monster yet it has 2.85 gear. Why? The 10 speed trans, the 10 forward gears eliminates the need for a deep rear gear and keep the engine in the meat of the torque curve through the pull, the very same thing the ZF 8 speed does in these trucks.
The rear deeper gear ratio's and be replicated by a deep first gear in the trans then slowly eliminated through the use of multiple forward gears

Exactly. The 8 speed zf with the 3.21 in first, has a higher gear ratio (better/easier pulling) than the toyota tundra 6 speed with a 4.30. It also manages to have a slightly lower final gear ratio for highway MPG. So on both ends of the spectrum, the 3.21 beats the toyota 4.30 for both pulling power, and for mpg crusing.

For all those disbelievers of the 3.21, let that sink in......

 

[HSKR R/T]

Exactly. The 8 speed zf with the 3.21 in first, has a higher gear ratio (better/easier pulling) than the toyota tundra 6 speed with a 4.30. It also manages to have a slightly lower final gear ratio for highway MPG. So on both ends of the spectrum, the 3.21 beats the toyota 4.30 for both pulling power, and for mpg crusing.

For all those disbelievers of the 3.21, let that sink in......

Why are we trying to compare to Toyota?

 

[HSKR R/T]

No, Im very well aware of how much the trans plays a role in this. My hobby is racing, the ZL1 camaro with the M6 is a dog, th ZL1 with the 10spd auto is a monster yet it has 2.85 gear. Why? The 10 speed trans, the 10 forward gears eliminates the need for a deep rear gear and keep the engine in the meat of the torque curve through the pull, the very same thing the ZF 8 speed does in these trucks.
The rear deeper gear ratio's and be replicated by a deep first gear in the trans then slowly eliminated through the use of multiple forward gears

You're missing my point and getting too hung up on available gear ratios, and not looking at the big picture of the shift schedule for transmission and still assuming the trans will magically always keep the transmission in different gears to "equalize" between a 3.92 truck and a 3.21 truck

 

[silver billet]

Why are we trying to compare to Toyota?

Because the Toyota has such a, uh "legendary" reputation for towing. Lot of guys who own one, first word out of their mouth, "it's so powerful" yada yada.

And it does tow well. But as good as it is, the 3.21 does better.

So I'm showing you that what used to be required for competent towing, is no longer as much of a requirement. If you have 6, 5 or heaven forbid, 4 gears, then rear end becomes more critical. 8 or 10, it becomes very unneccesary. The transmission itself is geared lower, and it can do that because it has so many gears to choose from.

So yeah, some of you guys are just stuck in the past and not able to understand why the 3.92 is no longer needed as much. You take what used to be a gospel truth (and rightly so), and apply that still to this 8 speed without understanding the math and why it's no longer needed.

 

[silver billet]

You're missing my point and getting too hung up on available gear ratios, and not looking at the big picture of the shift schedule for transmission and still assuming the trans will magically always keep the transmission in different gears to "equalize" between a 3.92 truck and a 3.21 truck

It's not magic. Example:

The 3.92 is in 7th gear, doing, say 1300 RPMs; do the math, the gear ratio for the 3.92 in 7th gear = 3.2144

Now check the available gear ratios for the 3.21 rear end. We have these options:
5th gear = 4.205
6th gear = 3.21
7th gear = 2.6322

If the transmission picks 6th gear, it will have an identical RPM as the 3.92, at the exact same speed.

Logically; why would the transmission pick 5th (and rev at 1800 rpms), or 7th (and rev at 800 - 900 rpms)?

Of course it's going to pick the same gear ratio (not the same "gear", but the same "gear ratio") that the 3.92 is using for the exact same mph and throttle input. Why the heck wouldn't it? They are both tuned to give the lowest RPMs it can offer, while still maintaining enough power as required by the throttle input and load on the truck.

 

[SpeedyV]

It's not magic. Example:

The 3.92 is in 7th gear, doing, say 1300 RPMs; do the math, the gear ratio for the 3.92 in 7th gear = 3.2144

Now check the available gear ratios for the 3.21 rear end. We have these options:
5th gear = 4.205
6th gear = 3.21
7th gear = 2.6322

If the transmission picks 6th gear, it will have an identical RPM as the 3.92, at the exact same speed.

Logically; why would the transmission pick 5th (and rev at 1800 rpms), or 7th (and rev at 800 - 900 rpms)?

Of course it's going to pick the same gear ratio (not the same "gear", but the same "gear ratio") that the 3.92 is using for the exact same mph and throttle input. Why the heck wouldn't it? They are both tuned to give the lowest RPMs it can offer, while still maintaining enough power as required by the throttle input and load on the truck.

The first post in this thread covers the gear ratios (perhaps negating the need for 17 pages of responses, but I digress). However, I'll concede that I haven't seen shift mapping factored into the discussion, per comments by @HSKR R/T. In reality, FCA states that there are 45 different shift maps for this transmission, and it "learns" (i.e. selects a specific map) based on real-world usage.

You assume that "...it's going to pick the same..." characteristics, but that's an idealized assumption. You (and I) don't have access to the algorithm. So it is possible that behavior matches under given load, rotational speed, and throttle input, but you (and I) can't prove that.

Likewise, it is possible that additional load factors are considered that enable the 3.92 to handle a more aggressive shift mapping, but @HSKR R/T can't prove that either.

Presumably, FCA tested the shift mapping with both rear end ratios under a wide variety of conditions...or we might end up with some pretty bizarre shifting patterns. And it does make sense that the engineers chose two rear end ratios that matched up most of the gear ratios (albeit one gear apart), as this simplifies the programming differences (really only required to respond differently in off-the-line and perhaps highway cruise behavior).

 

[dean77]

Just ordered a 2021 limited. Chose the 3.92’s based on driving 2 2020 big horns one a 3.21 one a 3.92. towing will be minimal. i Have read this thread numerous times and obviously great arguments both ways. But in the end i went with my feel and the 3.92 was stronger in the 2 areas I wanted, from a stop and secondary road passing. Now to see how it does in northern NH snow and ice!

 

[silver billet]

The first post in this thread covers the gear ratios (perhaps negating the need for 17 pages of responses, but I digress). However, I'll concede that I haven't seen shift mapping factored into the discussion, per comments by @HSKR R/T. In reality, FCA states that there are 45 different shift maps for this transmission, and it "learns" (i.e. selects a specific map) based on real-world usage.

You assume that "...it's going to pick the same..." characteristics, but that's an idealized assumption. You (and I) don't have access to the algorithm. So it is possible that behavior matches under given load, rotational speed, and throttle input, but you (and I) can't prove that.

Likewise, it is possible that additional load factors are considered that enable the 3.92 to handle a more aggressive shift mapping, but @HSKR R/T can't prove that either.

Presumably, FCA tested the shift mapping with both rear end ratios under a wide variety of conditions...or we might end up with some pretty bizarre shifting patterns. And it does make sense that the engineers chose two rear end ratios that matched up most of the gear ratios (albeit one gear apart), as this simplifies the programming differences (really only required to respond differently in off-the-line and perhaps highway cruise behavior).

The point is; the transmission is the same regardless of rear end. It's going to shift with the same strategies both ways.

And since you guys are really grasping at straws and will keep banging on about "assumptions"; the driver still has final say in the shifting. If I'm towing a trailer, beyond first and second gear, I can emulate the same behaviour and make my truck drive in 6th to match the same power and RPMs as the 3.92 in 7th.

(yeah yeah, "fiddling with the gear limiter again". horrors.)

 

[SpeedyV]

The point is; the transmission is the same regardless of rear end. It's going to shift with the same strategies both ways.

And since you guys are really grasping at straws and will keep banging on about "assumptions"; the driver still has final say in the shifting. If I'm towing a trailer, beyond first and second gear, I can emulate the same behaviour and make my truck drive in 6th to match the same power and RPMs as the 3.92 in 7th.

(yeah yeah, "fiddling with the gear limiter again". horrors.)

I’m not grasping at anything. I respectfully pointed out that while the math is straightforward, WE don’t necessarily have all the answers.

Much like @dean77 states above, I opted for the 3.92 because I preferred the off-the-line driving experience. I could care less which gear I’m in...we have plenty to choose from when needed.

 

[BowDown]

You're missing my point and getting too hung up on available gear ratios, and not looking at the big picture of the shift schedule for transmission and still assuming the trans will magically always keep the transmission in different gears to "equalize" between a 3.92 truck and a 3.21 truck

No I'm not missing anything. The trans does not shift by mile an hour, it shifts by throttle load, TPS and engine load detected by the map sensor.
Because of this, yes the PCM will keep the transmission in the appropriate gear based on load in speed Are you thinking that the trans has the same shift from a file for an 3.92 gear that it does for 3.21 gear?.

 

[BowDown]

Just ordered a 2021 limited. Chose the 3.92’s based on driving 2 2020 big horns one a 3.21 one a 3.92. towing will be minimal. i Have read this thread numerous times and obviously great arguments both ways. But in the end i went with my feel and the 3.92 was stronger in the 2 areas I wanted, from a stop and secondary road passing. Now to see how it does in northern NH snow and ice!

Initial 1st gear acceleration, you'll get what you're after. Stronger passing, no. The 3.21 truck will simply go to say 4th where the 3.92 truck goes to 5th and looking at post one, the EFFECTIVE ratio is the same

 

[silver billet]

I’m not grasping at anything. I respectfully pointed out that while the math is straightforward, WE don’t necessarily have all the answers.

Much like @dean77 states above, I opted for the 3.92 because I preferred the off-the-line driving experience. I could care less which gear I’m in...we have plenty to choose from when needed.

I'm sorry but "we don't have the answers" is completely false. The math doesn't lie.

Read (for example) dean77's recent post again. I mean it's completely obvious the math is escaping 99% of you guys.

 

[dean77]

Initial 1st gear acceleration, you'll get what you're after. Stronger passing, no. The 3.21 truck will simply go to say 4th where the 3.92 truck goes to 5th and looking at post one, the EFFECTIVE ratio is the same

I understand the effective ratio is the same. My feedback was the overall driving experience with the 3.92’s. The truck just felt more responsive to me across the board. Maybe it’s the mountains of northern NH I don’t know. I also understand that if I tow the 3.92’s will be a benefit, if I go with larger tires the 3.92’s again will be a benefit. The only negative I can see is an mpg hit but the more enjoyable driving experience, at least in my experience, is well worth it.

 

[BowDown]

I understand the effective ratio is the same. My feedback was the overall driving experience with the 3.92’s. The truck just felt more responsive to me across the board. Maybe it’s the mountains of northern NH I don’t know. I also understand that if I tow the 3.92’s will be a benefit, if I go with larger tires the 3.92’s again will be a benefit. The only negative I can see is an mpg hit but the more enjoyable driving experience, at least in my experience, is well worth it.

Given that the effective gear ratio after 2nd gear is almost identical for either gear, thats not possible.
1st and 2nd, yes. 3rd through 8th, the same.

 

[BowDown]

I'm sorry but "we don't have the answers" is completely false. The math doesn't lie.

Read (for example) dean77's recent post again. I mean it's completely obvious the math is escaping 99% of you guys.

I think that there's a "want" for the 3.92 gear set to deliver superior performance and if it were being used with a 4 or 5spd trans, they'd be correct. Despite post 1 and the math, many are not getting what the 8 spd trsns actually accomplishes.