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

[Robthebod]

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.

Thanks for that. Although I’m a dummy to all those ratios you posted, I get the jest of it. 3.21 better for gas mileage, 3.92 better for towing. Looks like not a huge difference though? I just purchased a new 2021 Laramie crew 4x4 and discovered that I have the 3.92. I rarely tow. Less than 1000 miles so far and computer says 16.5 mpg. Manuel check reveals actual 15.7. I know it’ll get better as it breaks in further. Here’s hoping it gets a lot better lol! In any case, I love that truck!

 

[Cliffrunner88]

Yeah really hoping it breaks in more. Almost 600 miles and it still says 13.X mpg.

 

[Klicht87]

Some of your mileage reports are kind of crazy. I have 3.92s and I can get around 19-20 on the highway and around 17 city driving. I will say I'm pretty light on the pedal until I need to gun it (only because of my awesome Borla ATAK exhaust).

 

[KWKSLVR]

Fuel Economy is apparently a dying art. I can jump in my mom's Grand Cherokee and coax 28-30MPG out of it on back roads and 33MPG on the highway. I can squeeze 27MPG out of my 4 cylinder Camry in town, or I can drive it like I stole it and get 22MPG. I get a kick out of it and it's penance for all the stupid driving I used to do in sports cars in my teens and 20's. The way I see the average truck driver drive around me, there is ZERO chance any of them are getting anything remotely close to EPA numbers. If you're going to drive 80mph and weave in and out of traffic like you're driving a Corvette, your mileage is going to suck. You're driving a brick at high speeds, every move you do that isn't done with fuel efficiency in mind compounds your fuel consumption the faster you go. I see the same complaint all over the internet, "I only get 17MPG, but I drive 80 on the interstate...so maybe that has something to do with it". Umm, yes. Our minivan's economy drops big time at 80MPG vs 70MPG.

 

[nc_beagle]

Fuel Economy is apparently a dying art. I can jump in my mom's Grand Cherokee and coax 28-30MPG out of it on back roads and 33MPG on the highway. I can squeeze 27MPG out of my 4 cylinder Camry in town, or I can drive it like I stole it and get 22MPG. I get a kick out of it and it's penance for all the stupid driving I used to do in sports cars in my teens and 20's. The way I see the average truck driver drive around me, there is ZERO chance any of them are getting anything remotely close to EPA numbers. If you're going to drive 80mph and weave in and out of traffic like you're driving a Corvette, your mileage is going to suck. You're driving a brick at high speeds, every move you do that isn't done with fuel efficiency in mind compounds your fuel consumption the faster you go. I see the same complaint all over the internet, "I only get 17MPG, but I drive 80 on the interstate...so maybe that has something to do with it". Umm, yes. Our minivan's economy drops big time at 80MPG vs 70MPG.

I would think it would be higher at 80mpg vs 70mpg.

Truck drivers are mixed around me. Some fast, some not. It sometimes seems to be correlated with how much lift they have on their trucks, with the more lift being the faster, more aggressive drivers.

 

[Mountain Whiskey]

Yep. I didn't buy this thinking it was a gas sipper. Cracks me up to hear that people are surprised at the milage. I have a lift and 35 jnch tires. I am not a slow driver by a long shot. 75 to 80 is common on my drive to work (at least since CV19 and the government paying people to stay home and off the roads). I see a bit over 15 mpg. It's OK.

 

[Redfour5]

Yeah really hoping it breaks in more. Almost 600 miles and it still says 13.X mpg.

I'm at 700 and 12.8. But I am taking them at their word on break in and stomping on it to get the RPM's up during break in. "

"A long break-in period is not required for the engine
and drivetrain (transmission and axle) in your
vehicle.
Drive moderately during the first 300 miles
(500 km). After the initial 60 miles (100 km),
speeds up to 50 or 55 mph (80 or 90 km/h) are
desirable.
While cruising, brief full-throttle acceleration within
the limits of local traffic laws contributes to a good
break-in. Wide-open throttle acceleration in low
gear can be detrimental and should be avoided."

 

[KWKSLVR]

All that break in is pretty much done by the 100 mile mark.

 

[Cliffrunner88]

All that break in is pretty much done by the 100 mile mark.

Well that's not good, if that means that this is my mpg now, lol.

 

[brian42]

I was running consistently in the low 14's until about 10K miles then I bumped up about 1 mpg over time after that.

Now it's based on fuel. I get about 1 mpg lower with winter blend.

 

[Redfour5]

I was running consistently in the low 14's until about 10K miles then I bumped up about 1 mpg over time after that.

Now it's based on fuel. I get about 1 mpg lower with winter blend.

I noticed that on my Renegade... All of a sudden it jumped a couple of MPG. I was hoping the new truck would also...as the mileage ratings have no relationship to what I am getting here new...

 

[HSKR R/T]

I was running consistently in the low 14's until about 10K miles then I bumped up about 1 mpg over time after that.

Now it's based on fuel. I get about 1 mpg lower with winter blend.

My Built to Serve was the same way. Just over 10K miles and the fuel mileage increased by 1-2mpg average.

 

[SpeedyV]

Well that's not good, if that means that this is my mpg now, lol.

It also adapts power delivery and shifting behavior to your driving style. You are apparently prioritizing performance over economy

 

[Cliffrunner88]

It also adapts power delivery and shifting behavior to your driving style. You are apparently prioritizing performance over economy

No I’ve been trying to get it up. Definitely not throttling it.
My current MPG in the cluster shows 18-20 while I’m cruising but the big MPG numbers in the middle are still showing 13.X. I just refueled this morning and reset my Trip A and it hasn’t changed. Am I not resetting something correctly? What is that big MPG number in the cluster really representing?

 

[IvoryHemi]

No I’ve been trying to get it up.

That sounds like a different issue.

 

[BowDown]

No I’ve been trying to get it up. Definitely not throttling it.
My current MPG in the cluster shows 18-20 while I’m cruising but the big MPG numbers in the middle are still showing 13.X. I just refueled this morning and reset my Trip A and it hasn’t changed. Am I not resetting something correctly? What is that big MPG number in the cluster really representing?

Post a pic of your cluster arrangement

 

[brian42]

My current MPG in the cluster shows 18-20 while I’m cruising but the big MPG numbers in the middle are still showing 13.X. I just refueled this morning and reset my Trip A and it hasn’t changed. Am I not resetting something correctly? What is that big MPG number in the cluster really representing?

Post a pic of your cluster arrangement

It sounds like the "big" MPG number in your cluster is coming from the fuel economy page, which is different than the trip odometer reading. In addition to resetting the trip odometer (pressing OK) you should go up one page to the fuel economy page and press OK for a few seconds and reset those calculations too.

 

[KWKSLVR]

It sounds like the "big" MPG number in your cluster is coming from the fuel economy page, which is different than the trip odometer reading. In addition to resetting the trip odometer (pressing OK) you should go up one page to the fuel economy page and press OK for a few seconds and reset those calculations too.

I'm loving all these pro tips I'm learning about.

 

[Cliffrunner88]

It sounds like the "big" MPG number in your cluster is coming from the fuel economy page, which is different than the trip odometer reading. In addition to resetting the trip odometer (pressing OK) you should go up one page to the fuel economy page and press OK for a few seconds and reset those calculations too.

Post a pic of your cluster arrangement

Ok thanks for the tip! I didn’t know I had to do that too. It’s just a “third” mpg trip?

ECO, non-ECO, and a different speed

 

[323F2R]

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.

Very nice write up. I have the 2020 1500 Classic, 392 ( I mean to say 5.7 Hemi — confused as my first Hemi was the 392 hemi of my 1956 St Regis), 3.21, 2wd.

Towing my 5000 lb trailer seems best in 6th gear which I need to select manually, as the tow mode typically runs under or at 2000 rpm at 57 mph. Where can I find a torque curve for this setup? What would you suggest as the optimum cruising rpm As the cruise control selects 7th ?

Appreciate your insight.