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

[Ramjack]

Everyone knows 3.92's contribute to global warming. You guys should be ashamed.

 

[Idahoktm]

That's more true than you may realize because the speedo will read artificially high as well since it is based on rotations and not actual ground speed.

Not if you reprogrammed the BCM with the new tire size.

 

[BluegrassMotorsport]

I've had my 2022 Ram 1500 with the 3.92 for about 1500 miles now. My first choice was the 3.21 since I very rarely tow but I found a killer deal on this truck that I wasn't about to pass up. What I notice more than anything is how high the rpms are on the highway, which I expected. In a flat land area, I'd say the 3.21 would be a lot easier on fuel. But I live in a very hilly area and honestly I don't think it makes much of a difference for me. Running the higher rpms in the 3.92 means it's shifting gears a lot less. Plus I can keep it in Eco mode a lot longer without cruise control. I average 17-18mpg overall which is pretty darn good considering the terrain I live in.

My last truck was a Silverado with the 6.2 and 10 speed. It was more powerful at 420hp and 460 torque. MPGs were much better on the highway but still averaged about the same overall, while paying for 93 vs 87 in the Ram. So my highway MPGs are lower in exchange for saving $0.60 a gallon on fuel. All-in-all it's about the same. Not a huge difference.

 

[KJ6MTJ]

I'm a bit late on this read, however, thank you for providing the charts with 3.21 vs 3.92. I myself have a base 2019 Bighorn 5.7 with a payload of 1788 and I lug around a 2021 Forest River Wildwood 24RLXL that grosses 7500LB and I have the 3.21 gears which I went onto the build sheet and confirmed a few items, my max tow is 8500 LBS. I average between 9.5 mpg to 10 mpg lugging that RV trailer around. I mainly use 6th at the highest top gear, 7th, its feels a little underpowered. Pretty much the longest tow I do is from Alabama to Texas. 3.21 work for me and this current trailer. Then again I don't mind hearing my engine run between 2,500 at 70 mph in 6th due to having a mild Flowmaster.

 

[ChrisID]

I've had my 2022 Ram 1500 with the 3.92 for about 1500 miles now. My first choice was the 3.21 since I very rarely tow but I found a killer deal on this truck that I wasn't about to pass up. What I notice more than anything is how high the rpms are on the highway, which I expected. In a flat land area, I'd say the 3.21 would be a lot easier on fuel. But I live in a very hilly area and honestly I don't think it makes much of a difference for me. Running the higher rpms in the 3.92 means it's shifting gears a lot less. Plus I can keep it in Eco mode a lot longer without cruise control. I average 17-18mpg overall which is pretty darn good considering the terrain I live in.

My last truck was a Silverado with the 6.2 and 10 speed. It was more powerful at 420hp and 460 torque. MPGs were much better on the highway but still averaged about the same overall, while paying for 93 vs 87 in the Ram. So my highway MPGs are lower in exchange for saving $0.60 a gallon on fuel. All-in-all it's about the same. Not a huge difference.

The mpg issue of 3.2 vs 3.9 has been opposite of what you would expect, for me. My 3.2 is in the mid-atlantic, small but constant hills. My 3.9 is in the Northwest, larger less frequent hills. My 3.9 has been getting 1-3mpg better across the board. Keep in mind that I am a heavy foot. I enjoy and use the power avail. Coming from GM's, it has been enjoyable to actually have trans tuning that I enjoy, and expect.
I recently did an experiment on the 3.2 where I babied the hell out of it for a whole tank (33gal), and I was able to achieve around 1-2 mpg better than the 3.9. But I mean really baby, with acceleration like a Prius. But if I drive like I want too, the 3.9 wins in every category (for me).
On a side note, a friend has a 3.9 in the MId-atlantic area and his 3.9 also gets 1-3 better than my 3.2.
I can see why Ram had to put 3.2's in as std., to help them meet CAFE, but that's babying it to the absurd.
I think 3.5-3.6's would have been a very good compromise as a standard gear for these trucks.

 

[KWKSLVR]

The mpg issue of 3.2 vs 3.9 has been opposite of what you would expect, for me. My 3.2 is in the mid-atlantic, small but constant hills. My 3.9 is in the Northwest, larger less frequent hills. My 3.9 has been getting 1-3mpg better across the board. Keep in mind that I am a heavy foot. I enjoy and use the power avail. Coming from GM's, it has been enjoyable to actually have trans tuning that I enjoy, and expect.
I recently did an experiment on the 3.2 where I babied the hell out of it for a whole tank (33gal), and I was able to achieve around 1-2 mpg better than the 3.9. But I mean really baby, with acceleration like a Prius. But if I drive like I want too, the 3.9 wins in every category (for me).
On a side note, a friend has a 3.9 in the MId-atlantic area and his 3.9 also gets 1-3 better than my 3.2.
I can see why Ram had to put 3.2's in as std., to help them meet CAFE, but that's babying it to the absurd.
I think 3.5-3.6's would have been a very good compromise as a standard gear for these trucks.

This experience seems to be pretty standard after reading this entire thread trying to decipher different experiences. I think 3.5's-3.6's would have been a good idea too, at least if they were only going to offer 1 gear set. I'm a highly functioning 65 year old in a 41 year old body in the way I drive. I baby everything. I leave 5 minutes early to go everywhere and I love squeezing every ounce of gas out of my vehicles. 3.21 just made sense for my uses, especially living in a relatively flat place. If I did more towing or was likely to ever tow anything more than a 4 wheeler and utility trailer I would have done 3.92's.

 

[Scram1500]

Why they don't offer the 3.55 with the Hemi is a total mystery. It's available for the V6 and the only option for the TRX, engines at either end of the spectrum

 

[KWKSLVR]

It probably has something to do with optimization. It very well could be that a 3.55 is not optimal for fuel efficiency or for towing, so instead they offer what they offer to let the consumer decide where they want to go to get the best of what they're looking for. For the TRX, it's probably optimal for overall performance and for the V6 a 3.92 gear set may very well give you worse fuel economy than it would with a Hemi the way the V6 may be tuned for it's power delivery (that gear set may rip you through the RPM range so fast you're giving it more gas to try and create more power because you're not in the right RPM range long enough).

That said, I'm not an engineer, but I do have lots of friends that are that would do the math on this stuff in college when we were endlessly wrenching on cars before the next track trip.

 

[AngelPhoenix]

It probably has something to do with optimization. It very well could be that a 3.55 is not optimal for fuel efficiency or for towing, so instead they offer what they offer to let the consumer decide where they want to go to get the best of what they're looking for. For the TRX, it's probably optimal for overall performance and for the V6 a 3.92 gear set may very well give you worse fuel economy than it would with a Hemi the way the V6 may be tuned for it's power delivery (that gear set may rip you through the RPM range so fast you're giving it more gas to try and create more power because you're not in the right RPM range long enough).

That said, I'm not an engineer, but I do have lots of friends that are that would do the math on this stuff in college when we were endlessly wrenching on cars before the next track trip.

This makes sense to me. 3.55's wouldn't really give something as "light" as a half-ton truck the towing capability bump it needs, nor would it make enough of a difference in increased fuel economy to be worth it for the bump in towing capability it hardly gives you.

I feel like I see more HD build with the 3.55's because 3/4+ tons can use every little bit of extra fuel economy they can get, and can probably make better use of the that gear with their base higher level of HP and torque than the 1/2 tons can. That's just a lot of theorizing on my part though.

 

[IvoryHemi]

I feel like I see more HD build with the 3.55's because 3/4+ tons can use every little bit of extra fuel economy they can get.....That's just a lot of theorizing on my part though.

Fuel Economy isn’t rated on HD trucks by the EPA. So mpg isn’t a manufacturer concern for CAFE

Ford is the only one offering 3.55 standard, (4.30 optional) while both Ram/GM are 3.73 standard on gas

 

[AngelPhoenix]

Fuel Economy isn’t rated on HD trucks by the EPA. So mpg isn’t a manufacturer concern for CAFE

Ford is the only one offering 3.55 standard, (4.30 optional) while both Ram/GM are 3.73 standard on gas


View attachment 114298

Right, still middleground gears though, so I wonder if that still plays with my theory at all. That's really funny though, I didn't know HD trucks weren't rated. Just, "You don't want to know. It's bad."

 

[Mountain Whiskey]

The mpg issue of 3.2 vs 3.9 has been opposite of what you would expect, for me. My 3.2 is in the mid-atlantic, small but constant hills. My 3.9 is in the Northwest, larger less frequent hills. My 3.9 has been getting 1-3mpg better across the board. Keep in mind that I am a heavy foot. I enjoy and use the power avail. Coming from GM's, it has been enjoyable to actually have trans tuning that I enjoy, and expect.
I recently did an experiment on the 3.2 where I babied the hell out of it for a whole tank (33gal), and I was able to achieve around 1-2 mpg better than the 3.9. But I mean really baby, with acceleration like a Prius. But if I drive like I want too, the 3.9 wins in every category (for me).
On a side note, a friend has a 3.9 in the MId-atlantic area and his 3.9 also gets 1-3 better than my 3.2.
I can see why Ram had to put 3.2's in as std., to help them meet CAFE, but that's babying it to the absurd.
I think 3.5-3.6's would have been a very good compromise as a standard gear for these trucks.

Yes, you are experiencing the simple fact that milage is not all about engine rpm but really how hard the engine works. Uneven terrain, oversized tires hauling more than the kid in the back seat all attribute to the motor working harder with 3:21 gears. Fine if you have bicycle tires onthe truck and only drive downhill with the wind at your back and are using expensive gas so it does not ping each time you touch the gas pedal. Regular truck stuff? Stick with better gears.

 

[Ramjack]

Yes, you are experiencing the simple fact that milage is not all about engine rpm but really how hard the engine works. Uneven terrain, oversized tires hauling more than the kid in the back seat all attribute to the motor working harder with 3:21 gears. Fine if you have bicycle tires onthe truck and only drive downhill with the wind at your back and are using expensive gas so it does not ping each time you touch the gas pedal. Regular truck stuff? Stick with better gears.

The concept of an engine working harder (i.e. less efficiently) is somewhat counterintuitive. A vehicle requires a certain amount of horsepower to be accelerated from a stop to a certain speed at a certain rate of acceleration. Let's say it takes 100hp for purposes of this example. The setup (i.e. gear ratio) that results in the engine operating with a more open throttle plate will be the more efficient setup. This means the setup that results in the engine operating at lower rpm (and therefore wider throttle opening) will be more efficient. Contrary to popular belief, engines are not working harder when they generate 100hp at 2000 rpm at a 25% throttle opening (call this "Engine A") instead of generating 100hp at 3000 rpm with a 15% throttle opening (call this "Engine B"). Engine A is actually operating more efficiently because it has lower throttling losses. In other words, Engine A is generating more of the needed horsepower (100hp in this case) through torque instead of RPM. Yes, in extreme cases there can be downsides to this if you get to the point of "lugging" an engine over long periods of time. This is very hard on the bottom end components. But we are talking about fuel-efficiency here.

A simpler example (with fewer variables): Let's say you have an two identical boats with the exact same engines, but one has a 17" pitch propeller and the other has a 19" pitch. (This is the same as having a different gear ratios.) Say the boat requires 100hp to hold a constant speed of 35mph. Assume Boat A with a 17P propeller will hold a constant 35mph speed at 3800 rpm with a 50% throttle opening. Assume Boat B with a 19P propeller will hold a constant 35mph speed at 3400 rpm with a 60% throttle opening (a higher throttle opening is necessary for the engine to produce the same power at a lower rpm). Boat B will be more fuel-efficient because of lower throttling losses.

Having 8 speeds available certainly complicates this analysis and means there can certainly be exceptions. But it does not change the overall premise that a gasoline engine works more efficiently when it is generating the horsepower required of it using the lowest possible rpm (and therefore the highest possible throttle opening). Diesel engines on the other hand are an entirely different concept and I hope to better understand them someday. For the time being, over my head.

 

[ferraiolo1]

*insert popcorn emoji


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[Ramjack]

*insert popcorn emoji


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As long as it's not microwave popcorn. That stuff is NASTY.

 

[Mountain Whiskey]

My eyes started bleeding a little, I took a nap, continued and then got a headache and had to stop reading.

Basically though, with lower (higher numerical) gears, it does not require that 100hp to crank the tires and fight the wind that the wimpy 3:21 gears need. So yea, 3:92 for the win as always.

 

[nc_beagle]

The concept of an engine working harder (i.e. less efficiently) is somewhat counterintuitive. A vehicle requires a certain amount of horsepower to be accelerated from a stop to a certain speed at a certain rate of acceleration. Let's say it takes 100hp for purposes of this example. The setup (i.e. gear ratio) that results in the engine operating with a more open throttle plate will be the more efficient setup. This means the setup that results in the engine operating at lower rpm (and therefore wider throttle opening) will be more efficient. Contrary to popular belief, engines are not working harder when they generate 100hp at 2000 rpm at a 25% throttle opening (call this "Engine A") instead of generating 100hp at 3000 rpm with a 15% throttle opening (call this "Engine B"). Engine A is actually operating more efficiently because it has lower throttling losses. In other words, Engine A is generating more of the needed horsepower (100hp in this case) through torque instead of RPM. Yes, in extreme cases there can be downsides to this if you get to the point of "lugging" an engine over long periods of time. This is very hard on the bottom end components. But we are talking about fuel-efficiency here.

A simpler example (with fewer variables): Let's say you have an two identical boats with the exact same engines, but one has a 17" pitch propeller and the other has a 19" pitch. (This is the same as having a different gear ratios.) Say the boat requires 100hp to hold a constant speed of 35mph. Assume Boat A with a 17P propeller will hold a constant 35mph speed at 3800 rpm with a 50% throttle opening. Assume Boat B with a 19P propeller will hold a constant 35mph speed at 3400 rpm with a 60% throttle opening (a higher throttle opening is necessary for the engine to produce the same power at a lower rpm). Boat B will be more fuel-efficient because of lower throttling losses.

Having 8 speeds available certainly complicates this analysis and means there can certainly be exceptions. But it does not change the overall premise that a gasoline engine works more efficiently when it is generating the horsepower required of it using the lowest possible rpm (and therefore the highest possible throttle opening). Diesel engines on the other hand are an entirely different concept and I hope to better understand them someday. For the time being, over my head.

 

[Ramjack]

My eyes started bleeding a little, I took a nap, continued and then got a headache and had to stop reading.

Basically though, with lower (higher numerical) gears, it does not require that 100hp to crank the tires and fight the wind that the wimpy 3:21 gears need. So yea, 3:92 for the win as always.

I'm glad you used a static speed example. I should have stuck with that in my examples. In reality, the truck needs the same 100hp regardless of gear ratio to hold the truck at a constant speed X. What changes is the torque requirement. The truck will require about 18% less torque, but 22% more rpm to keep the truck moving at a constant X mph with the 3.92 gears vs 3.21 gears, assuming we're in the same transmission gear. This is because power has a time component (Power = Force x Distance / Time). (82%x122% = 100% so this brings the HP back to the same)

Honestly, I would rather have 3.92 gears for the acceleration, but I took what I could get in light of the deal I got. So I'm not trying to say 3.21's are better, not by any means.

 

[ferraiolo1]

I’ll just preemptively make a comment for mountainwhisky instead of using math and logic.

3.9 is a mans gear 3.2 is a sissy gear no matter the scenario!


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[Cda3488]

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.

Guess that’s why they put the 3.92 in the GT package, a little extra pep!