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

[Mountain Whiskey]

Candy, thats a strange way to spell dylan and you guys were hanging out? Hmmmm, I guess real men of genius

I can't even think of a comeback for that one!

 

[RVTRKN]

I'll be honest, I just read only the first two pages and jumped to pg# 17.

I just sold my 2019 3500 SRW CTD with 3:73's, I refused to purchase a CTD with 3:42's, and waited for the 2019. So when I bought my 2020 1500 Big Horn I was worried about the 3:21 rear diff's and was surprised with the power on takeoff with the 5.7 Hemi (non eTorque). I only tow a light pontoon now (5K lbs) and it has plenty of power towing it. The old argument about which ratio is best, will never go away. Its more what the manufacture equips them with, and what price you are willing to pay for, to get what you want. If I had a lot of hills or mountain driving, I'd go for 3:92 and get better MPG and 3:21 for flat roads like where I live. I'll apologize about this opinion in advance, but if your going to tow heavy to require the 3:92's, your buying the wrong truck, a 2500 would be your next level, not 3:92's.

 

[spyder]

The TRX doesn't have 3.92s does it?

https://twitter.com/i/web/status/1698524763994378240

 

[Eighty]

The TRX doesn't have 3.92s does it?

https://twitter.com/i/web/status/1698524763994378240

The TRX has the best of both worlds...3.55

 

[spyder]

The yellow one hauling that trailer out makes me laugh. Good for him.

 

[theblet]

The TRX has the best of both worlds...3.55

I thought it had 3.08s

 

[Eighty]

I thought it had 3.08s

No sir. But 4.10 would be nice...

 

[Mountain Whiskey]

The TRX doesn't have 3.92s does it?

https://twitter.com/i/web/status/1698524763994378240

Nope. It doesn't have a 5.7 N/A motor either!

 

[Mountain Whiskey]

The TRX has the best of both worlds...3.55

The best of both worlds... a 6.2l hemi and a blower!

Get em while you can! '24 is the last year.

 

[theblet]

The best of both worlds... a 6.2l hemi and a blower!

Get em while you can! '24 is the last year.

You go first brotha!

 

[Mountain Whiskey]

You go first brotha!

Nah, I'm busy making my sedan faster. I can make it go faster than I can a truck.

 

[spyder]

I'm a fan of NA big blocks. Because eff efficiency and rational thought.

 

[spyder]

Oh yeah, I see it in his signature. 9" more bed length has made the truck much more useful. As long as he thinks so, that's all that matters.

Finished a 1200 mile desert trek last week. Didn't have to stop to pour gas cans in between fuel stations like I did every time with the Rebel. Hell, still had a quarter tank when I got to the next fuel stop. Not having to haul additional fuel makes this truck much more useful.

Plus, now I can sleep in the back of it without my feet hanging off the end of the tailgate.

 

[Rick7812]

I hope this post will help to end the debate with facts and not opinions, and become THE post people refer to those who are having a hard time deciding.

You already know that 3.92 is better for towing, and 3.21 gets better fuel economy, so I will talk about what you might not know

Bottom line up front:

In layman's terms, to conceptualize the difference, imagine

1) 5 out the of 8 gears have the same final drive ratio between 3.21 and 3.92.

2) 3.21 has "an extra" overdrive gear.

3) 3.21 has 2 unique lower gears for towing.

4) 3.92 has 3 unique lower gears for towing.

5) Speed range that 3.21 is better at towing: 31-38 MPH, 48-57 MPH.

6) Speed range that 3.92 is better at towing: 0-30 MPH, 39-47MPH, 58-70 MPH.


Explanation


1) 5 out the of 8 gears have the same final drive ratio between 3.21 and 3.92:

Here's the gear ratio for the 8 speed transmission:
1) 4.71:1 2) 3.14:1 3) 2.10:1 4) 1.67:1 5) 1.29:1 6) 1.00:1 7) 0.84:1 8) 0.67:1 Reverse) 3.30:1

Final drive ratios with 3.21

1st. 15.12, 2nd. 10.10, 3rd. 6.74, 4th. 5.36, 5th. 4.14, 6th. 3.21, 7th. 2.70, 8th. 2.15, R 10.6

Final drive ratios with 3.92

1st. 18.46, 2nd. 12.31, 3rd. 8.23, 4th. 6.55, 5th. 5.06, 6th. 3.92, 7th. 3.29, 8th. 2.62, R 12.94

From the list below, we can see that gears 3-7 in 3.21 matches gears 4-8 in 3.92:

-- NO MATCH -- = 18.46 - 1st - 3.92
3.21 - 1st - 15.12 = -- NO MATCH --
-- NO MATCH -- = 12.31 - 2nd - 3.92
3.21 - 2nd - 10.1 = -- NO MATCH --
-- NO MATCH -- = 8.23 - 3rd - 3.92
3.21 - 3rd - 6.74 = 6.55 - 4th - 3.92
3.21 - 4th - 5.36 = 5.06 - 5th - 3.92
3.21 - 5th - 4.14 = 3.92 - 6th - 3.92
3.21 - 6th - 3.21 = 3.29 - 7th - 3.92
3.21 - 7th - 2.70 = 2.62 - 8th - 3.92
3.21 - 8th - 2.15 = -- NO MATCH --

2) 3.21 has "an extra" overdrive gear:

The 8th gear in 3.92 is the 7th gear in 3.21, thus effectively mean the 8th gear in the 3.21 is an extra gear to the 3.92.

Meaning, when you go test drive the 3.21 you will have to downshift to 7th to get the same acceleration at 3.92's 8th on freeways. That is why some people complain about how "sloppy" the 3.21 is, because the 3.21 has an extra overdrive gear for fuel economy. If you shift 3.21 in 7th gear, you will get the same acceleration as the 3.92 in 8th on the freeway. No, 3.21 isn't sloppy, you're just in a gear that 3.92 does not have.

3) 3.21 has 2 unique lower gears for towing:

As we know from 1), 5 gears have the same final drive ratio.
You "gain" an overdrive gear, but you "lose" one towing gear.
Here's the final drive ratio for the 2 towing gears.
1st. 15.12, 2nd. 10.10,

4) 3.92 has 3 unique lower gears for towing:

Same logic as the last
Final drive for 3 towing gears.
1st. 18.46, 2nd. 12.31, 3rd. 8.23.

5) Speed range where 3.21 is better at towing: 31-38 MPH, 48-57 MPH,
AND
6) Speed range where 3.92 is better at towing: 0-30 MPH, 39-47MPH, 58-70 MPH:

Calculated towing shift point to be 6000 rpm, if I'm off the logic is the same but the speed will vary.

For towing,
From the speed 0-30 MPH, 3.92 has higher final drive ratio over 3.21 (18.46 vs 15.12) until it has to shift to 2nd gear at 30MPH.

From the speed 31-38 MPH, 3.21 has higher final drive ratio over 3.92 (15.12 vs 12.31) until it has to shift to 2nd gear at 38MPH.

From the speed 39-47 MPH, 3.92 has higher final drive ratio over 3.21 (12.31 vs 10.10) until it has to shift to 3rd gear at 47 MPH.

From the speed 48-57 MPH, 3.21 has higher final drive ratio over 3.92 (10.10 vs 8.23) until it has to shift to 3rd gear at 57 MPH.

From the speed 58-70 MPH, 3.92 has higher final drive ratio over 3.21 (8.23 vs 6.74) until it has to shift to 4th gear at 70 MPH.

The key takeaway here is that towing heavier trailers uphill with 3.21 might never reach the desired speed within the 58-70 MPH range (typical highway towing speed) because 3.21 jumps from 10.10 to 6.74 without the 8.23 final drive ratio found in 3.92 that really help maintaining highway towing speed at max load.

Do you value the "extra" overdrive gear for fuel economy? or do you value the extra towing capability that you tell yourself you might one day need? That's up to you.

So my question is this. My Gen3 has a towing button that takes it out of overdrive. My gen5 has factory brake control. When I am towing does the truck recognize the fact and keeps me out of overdrive or do I have to use paddle shifter to stay in 7th gear??

 

[Dewey]

Finished a 1200 mile desert trek last week. Didn't have to stop to pour gas cans in between fuel stations like I did every time with the Rebel. Hell, still had a quarter tank when I got to the next fuel stop. Not having to haul additional fuel makes this truck much more useful.

Plus, now I can sleep in the back of it without my feet hanging off the end of the tailgate.

I’ll never buy another 5’7” bed. Seems everything I want to haul needs another 6”-9” of space to keep the tailgate closed. Really notice it when packing for hunting trips.
I checked out a 5’7” RamBox at a dealer and holy crap is that bed small. I get that you can pack small items in the compartments but your really restricted hauling bulky items in the bed. I use that extra space around the wheel wells often.

 

[Ramroo]

Finished a 1200 mile desert trek last week. Didn't have to stop to pour gas cans in between fuel stations like I did every time with the Rebel. Hell, still had a quarter tank when I got to the next fuel stop. Not having to haul additional fuel makes this truck much more useful.

Plus, now I can sleep in the back of it without my feet hanging off the end of the tailgate.

Kinda what I did.
Went from 3.92 short bed, to 3.21 long bed + topper. Love the setup now.

 

[HSKR R/T]

So my question is this. My Gen3 has a towing button that takes it out of overdrive. My gen5 has factory brake control. When I am towing does the truck recognize the fact and keeps me out of overdrive or do I have to use paddle shifter to stay in 7th gear??

Your Gen 3 needed to stay out of OD to not destroy the transmission while towing. The new ZF 8-speed is a better transmission. Hit the tow/haul button and let the truck do the rest.

 

[BowDown]

So my question is this. My Gen3 has a towing button that takes it out of overdrive. My gen5 has factory brake control. When I am towing does the truck recognize the fact and keeps me out of overdrive or do I have to use paddle shifter to stay in 7th gear??

No, changes upshifts and down shifts rpm and on etorque trucks, seems to use etorque to help in braking

 

[Mountain Whiskey]

Finished a 1200 mile desert trek last week. Didn't have to stop to pour gas cans in between fuel stations like I did every time with the Rebel. Hell, still had a quarter tank when I got to the next fuel stop. Not having to haul additional fuel makes this truck much more useful.

Plus, now I can sleep in the back of it without my feet hanging off the end of the tailgate.

Yea, a guy I know with 3.21 gears just went from DC to Alaska for a fishing trip, down to see the redwoods in CA, swung down through Vegas to hit the slots, popped in on a friend in Atlanta and still had more gas in the tank than when he left.

I'm telling you those 3.21 gears just make gas as you go down the road! Just call them Mercle Gears, I'll tell you what, mercles is jus what they are.

 

[Rick3478]

So my question is this. My Gen3 has a towing button that takes it out of overdrive. My gen5 has factory brake control. When I am towing does the truck recognize the fact and keeps me out of overdrive or do I have to use paddle shifter to stay in 7th gear??

Interesting question, and I don't know the answer. But they could have done that in software, since the controller recognizes when there are trailer brakes attached. Could have even made it user configurable with a separate max gear setting for each trailer type. Like we don't have enough settings to dig for. If I had to bet on it I'd say they probably didn't, and the up/down buttons or paddles will have to suffice.