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

[2020RamSport]

Ram needs to offer the 3.55s with the Hemi!

 

[Gen3CoupeTX]

No way to know if it's true, but the dealer I bought my 2019 from said nobody had ever said, "Must have heated front seats (cloth) and 3.92 rear." Heated seats for my wife, ratio for me. No particular reason, as I'd rarely if ever tow ... I just prefer the drive and responsiveness. We old-school types remember 3 and 4 speed transmissions, where selecting gears like 4.11 or 4.56 was cool around town, but miserable on highways. No longer an issue!

 

[Nails]

Ram needs to offer the 3.55s with the Hemi!

Agree

 

[BPA5150]

The 3.92 should have better acceleration than 3.21, but does anyone have any idea of the time difference from 0-60? Are there any actual numbers out there?

 

[Blackhawktech]

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. 3.29, 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. 3.21, 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.

I have the 3.21 gears in my 2019 ram 1500 and they pull fine. My issue with the 3.21 gears is the ability of the truck to stay in overdrive doing down the highway when running over 55 mph. Anytime I go up a slight hill mine shifts out of 8 th gear sometimes down to 6 th gear. This hurts your gas mileage. My truck averages 14.8 to 15 mpg. One time got 16.3 but only because I stayed below 60 mph and was in no traffic.

 

[Raydar]

I have 3.92s, and the best mileage I've seen (calculated at fill-up) was 16.9. The worst was in the 13s.
The worst included a lot of idling and stop-start driving on the farm. The best included a lot of steady speed driving (and a lighter foot, once the "novelty" of the Flowmaster wore off. )
(For comparison, my 2005 5.3 Silverado 2WD, with 3.23s, got ~15.)

 

[Lize]

Why would someone commenting on their choice - be considered an idiot?

Indeed, surely people commenting is a good thing and it shows the OP that supplying the information has been a useful exercise. If it is after purchase then they have gained peace of mind in their choice and thats a positive outcome.

 

[Jack]

I have the 3.21 gears in my 2019 ram 1500 and they pull fine. My issue with the 3.21 gears is the ability of the truck to stay in overdrive doing down the highway when running over 55 mph. Anytime I go up a slight hill mine shifts out of 8 th gear sometimes down to 6 th gear. This hurts your gas mileage. My truck averages 14.8 to 15 mpg. One time got 16.3 but only because I stayed below 60 mph and was in no traffic.

Situations that require you to shift to 6th would require 3.92 to shift to 7th. However situations where it requires the 3.21 to shift to 8th, the 3.92 does not have the equivalent gear to match. It would be like 3.21 is stuck in 7th gear. Now, towing is another story, 3.92 is a must for towing over 8000lbs.

 

[SpeedyV]

Situations that require you to shift to 6th would require 3.92 to shift to 7th. However situations where it requires the 3.21 to shift to 8th, the 3.92 does not have the equivalent gear to match. It would be like 3.21 is stuck in 7th gear. Now, towing is another story, 3.92 is a must for towing over 8000lbs.

While that is all true mathematically, it feels like an oversimplification. We’d have to map the power curve of the engine against these gear ratios to fully understand how the “sweet spot” varies for each, and to truly understand where the necessary input force relative to the powerband requires a downshift. Make sense?

 

[Chippy]

Indeed, surely people commenting is a good thing and it shows the OP that supplying the information has been a useful exercise. If it is after purchase then they have gained peace of mind in their choice and thats a positive outcome.

See I have been on other forums with most likely a younger demographic profile and refuse to get drawn into Keyboard hero stuff but others here I’m sure will continue.
If you couldn’t see others posting useless spouting to justify their choice and that their choice made them smarter than others or superior I’m sorry I did.
mom also to lazy to sit and type a novel on a forum.
the OP’s info should be a sticky and a lot of the rest of the thread could be an episode of the View with Whoopie and Joy

 

[Jack]

While that is all true mathematically, it feels like an oversimplification. We’d have to map the power curve of the engine against these gear ratios to fully understand how the “sweet spot” varies for each, and to truly understand where the necessary input force relative to the powerband requires a downshift. Make sense?

What makes sense is, it shifts to whichever gear the engineer programmed it to. Nobody knows if they’ve done the job right.

 

[SpeedyV]

What makes sense is, it shifts to whichever gear the engineer programmed it to. Nobody knows if they’ve done the job right.

True that!

 

[Chippy]

Why So Many Carmakers Use ZF's Eight-Speed Automatic

It's on-par with dual-clutch in terms of performance, but smoother, cheaper, and easier to package. It's no wonder so many cars use it.

www.roadandtrack.com

 

[HST]

Its the same trans thats in my 5 series BMW . As far as ratios i have 3.92 in my eco diesel with 285x65x20 and i get 21 around town . the 2000 ram that i just replaced had 4.10 s .
I think if you put 35 S on a truck with 3.21 it would be doggy from a stop never mind towing. But thats just my opinion.

 

[Willwork4truck]

Ram needs to offer the 3.55s with the Hemi!

Been saying that for awhile. Coming from Ford awhile back I could have chosen 3.55 or 3.73, made more sense. Same gears debate over there, FCA/RAM just made it farther on the extremes By dropping the 3.55 for the hemi. Id rather they dropped the 3.21 but cafe standards dictated otherwise.

 

[Willwork4truck]

I have the 3.21 gears in my 2019 ram 1500 and they pull fine. My issue with the 3.21 gears is the ability of the truck to stay in overdrive doing down the highway when running over 55 mph. Anytime I go up a slight hill mine shifts out of 8 th gear sometimes down to 6 th gear. This hurts your gas mileage. My truck averages 14.8 to 15 mpg. One time got 16.3 but only because I stayed below 60 mph and was in no traffic.

That was one reason I chose the hemi over the 6. The Pentstar 3.55 eco seemed like it downshifted when there was any accelerator used, merging traffic, or a slight incline dropped gears. At least the 3.21 hemi had the tq to not change them on my test loop. Never tried the 3.92 since I had read a few of the gear war threads before and figured I’d suffer with the 3.21.
IMHO I do think the choice is what do you want... nobody drives a pickup for gas mileage. (I came from a Prius and an F150).
This, oil type and gas octane discussions will never end. (Oh, and “can I tow this load/trailer“ questions.)

 

[Smerberj]

Awesome write up! I went with the 3.55 on the V6 and called it a day, no downshifting for me when using cruise control up a slight grade, actually impressed me.

 

[ColoradoCub]

Lots of variables that have to be considered.

Do you live in Florida or a Texas where the roads are flat or do you live in the Rockies where the road is constantly going uphill and downhill? Terrain affects performance both empty and loaded with loaded being affected even more by terrain.

Every size of tire installed taller and wider than the weenie tires the truck comes with will affect performance even more. I saw a reduction in RPMS at highway speeds by installing 34” tires right away which changes the gearing from 3.92. If you go with a taller tire than stock with 3.21’s your going to cause the truck to lug and hunt for gears even more which will affect performance.

There’s a lot to ponder on gearing choices for ones specific needs and geographical location.

 

[Grouper]

Thanks for the informative post. Has anyone composed a MPH vs. RPM chart for 3.21 vs. 3.92 with stock tires sizes?

My 2010 Silverado with 3.42 and 6 speed transmission turns 2000 RPM at 80 MPH on the GPS. From what I can gather the 3.92 8 speed combo will turn 2000 RPM at about 70 MPH.

If I were ordering, I'd order the 3.21. But there are a couple of trucks on nearby lots that are pretty close to configurations I could live with (for the right price) but both are 3.92's.

I really don't do a ton of highway driving but when I do, it's often at 80 mph.

 

[JustMe]

Amen to that ! VARIABLES can be as narrow or wide simply because of "personal taste" in wheels & tires. Then as you say. Location, Location, Location . . . ! What will I be towing, how often and for how much time (short or long distances), on highways or byways . . . this list could be much longer but my fingers won't last the distance .
"Personal intuition," without Factual insight will result in reinforcing false expectations (usually). Then again, "you" might be the lucky one and select the right gearing (for you), the right wheel/tire combo - that pleases the eye & the wallet & the trucks performance. Now THAT's a 3-fer we all can live with .

ME? I'll be adding a S.S. Cap (~275-lbs.) that will be a permanent fixture . I won't be changing wheels, but tires maybe, and they won't be Out of Specs (not smaller, perhaps 1/4" larger?). It's gonna be a "lighter (to start) Quad," but of course that Cap will bring me up to a Crew Cab weight in no time . . . which is one reason I want the Quad . The other less obvious reason, unless you know me, is I want/need the Space/length of a 6'4" bed - dispersed camping & all the gear needed . Plenty of room behind the front seats (in a quad) for a mid sized dog .
LIFT ? Maybe later - 2" Mopar (to avoid warrantee issues) - this will help with the off-road "Angles," but even then I will not increase wheel tire size (don't care IF it MAY look a little "off" - the dog won't care & neither will I; don't care what "you" may think . Oh, nearly forgot . . . my future RAM V6 will have the 3.55's . . . it just may help the truck get over some difficult situations in lower gears (crawling !). But in all likelihood it just gives me piece of mind . . . until it don't . . . ? . That's my story and I'm sticking to it, for now ! Geez, now my fingers really are sore, ha.