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

[SpeedyV]

So again, if your truck is able to run in 8th with ECO on, then my truck (3.21) will be able to run in 7th with ECO on. We would be running identical RPMs and identical MPG.

Well, if you really want to get technical about it, there are still differences. Yes, both trucks would be turning the same engine RPM. However, one truck is turning a small gear in the transmission, upstream from the transfer case in 4WD configurations, versus turning a larger gear in the rear differential to get to the same end state. So to presume identical MPG means we must ignore any mechanical mass effects and/or any parasitic driveline losses that might occur due to spinning the driveshaft at different speeds, even if the rear axle is turning at the same speed.

I'm way too lazy to do the math on this, but I would expect to find some 'edge cases' (specific RPM ranges for given gears) where the 3.92 is at least as efficient as the 3.21. But you've already stated the undisputed fact (at least undisputed by me) that a 3.21 will always operate with better average efficiency than a 3.92 in real-world conditions, perhaps barring extremes. And I own a 3.92 (for reasons that have nothing to do with efficiency)!

 

[silver billet]

Well, if you really want to get technical about it, there are still differences. Yes, both trucks would be turning the same engine RPM. However, one truck is turning a small gear in the transmission, upstream from the transfer case in 4WD configurations, versus turning a larger gear in the rear differential to get to the same end state. So to presume identical MPG means we must ignore any mechanical mass effects and/or any parasitic driveline losses that might occur due to spinning the driveshaft at different speeds, even if the rear axle is turning at the same speed.

I'm way too lazy to do the math on this, but I would expect to find some 'edge cases' (specific RPM ranges for given gears) where the 3.92 is at least as efficient as the 3.21. But you've already stated the undisputed fact (at least undisputed by me) that a 3.21 will always operate with better average efficiency than a 3.92 in real-world conditions, perhaps barring extremes. And I own a 3.92 (for reasons that have nothing to do with efficiency)!

Agreed; I think we (speaking for myself) sometimes do ignore the gearing advantage, but I also agree it's probably extremely negligible at 60+ mph, where our trucks are in 7'th or 8'th gear. If we were talking first gear, well, we know the advantage there goes quite heavily to the 3.92 since it can pull an extra 3000 pounds.

I just don't like seeing posts saying "the 3.92 is equal to or better than the 3.21 at highway speeds", without any math or anything resembling proof or stats or testing of some sort, because it's not better. It's better for getting off the line while towing max loads, it's not better for MPG.

And I'm willing to change my mind on the matter; heck I'd love to be wrong because it would give me another reason to trade in my truck which is always a fun thing to do. But all the MPG pics I've seen, with highway MPG greater than mine, have all been in 3.21 trucks. I personally have not seen one poster posting more than 20 MPG with the 3.92 (though of course I might have missed it).

 

[SpeedyV]

I just don't like seeing posts saying "the 3.92 is equal to or better than the 3.21 at highway speeds", without any math or anything resembling proof or stats or testing of some sort, because it's not better. It's better for getting off the line while towing max loads, it's not better for MPG.

Agreed. This brings us full-circle to the OP. From the start, @Jack affirmed that the 3.92 rear axle provides a towing advantage, while the 3.21 provides better economy. He also clearly explained why the 3.92 has a towing advantage in many situations, most notably off the line and at highway speeds. To your point, whenever we're talking about economy in the range of 20+ mpg, we're talking about an unloaded truck with 3.21s. When towing heavy loads, nobody's going to be winning awards for economy, but it's very plausible that the 3.92 will outshine the 3.21 on the highway (as the latter may be forced to upshift more frequently, thus losing its advantage).

 

[WXman]

As another follow up; somebody else in this forum much smarter than me, did the math and came up with the observation that the RPMS are pretty much equivalent between a 3.92 in 8th, vs a 3.21 in 7th.

So again, if your truck is able to run in 8th with ECO on, then my truck (3.21) will be able to run in 7th with ECO on. We would be running identical RPMs and identical MPG.

So your truck has no advantage over the 3.21 when you're in 8th. And, my truck has an additional gear which either does get better MPG in 8th, or at worst case I can run in 7th if needed and get the exact same MPG that you would. My truck has more possibilities and room for finding the best MPG at highway speeds.

I see what you're trying to say, but I still don't agree with it.

First of all, nobody starts out in 7th gear. And if you ever hopped onto a 10-speed bike as a kid, you know full well that it's easier to start peddling in a lower gear than it is a higher gear. So from a standing stop, the 3.21 truck will burn more fuel just getting up to speed.

Then, once up to speed, it's going to be more difficult for the truck to hold 8th gear with 3.21s, which creates more trans. shifting, which lowers fuel economy and builds heat in the fluid....which was the point of my last post.

So, in the real world, my experience* has been that numerically higher gears are ALWAYS an advantage with the exception of high speed driving on an open interstate. And since I (as well as the average guy) don't spend much of my time at high speed, the 3.92s were a no-brainer for mileage, towing, and general driving. You can lay out numbers on paper, but unfortunately the real world has variables that don't exist on that paper.

*By experience, I mean dozens of midsize and fullsize trucks and SUVs over the last 25 years, with axle ratios from 2.65 to 4.88, many of which I re-geared myself at home for the purpose of experimentation.

 

[silver billet]

I see what you're trying to say, but I still don't agree with it.

First of all, nobody starts out in 7th gear. And if you ever hopped onto a 10-speed bike as a kid, you know full well that it's easier to start peddling in a lower gear than it is a higher gear. So from a standing stop, the 3.21 truck will burn more fuel just getting up to speed.

Then, once up to speed, it's going to be more difficult for the truck to hold 8th gear with 3.21s, which creates more trans. shifting, which lowers fuel economy and builds heat in the fluid....which was the point of my last post.

So, in the real world, my experience* has been that numerically higher gears are ALWAYS an advantage with the exception of high speed driving on an open interstate. And since I (as well as the average guy) don't spend much of my time at high speed, the 3.92s were a no-brainer for mileage, towing, and general driving. You can lay out numbers on paper, but unfortunately the real world has variables that don't exist on that paper.

*By experience, I mean dozens of midsize and fullsize trucks and SUVs over the last 25 years, with axle ratios from 2.65 to 4.88, many of which I re-geared myself at home for the purpose of experimentation.

I'm sorry but you are completely incorrect. Unloaded/during commuting, the 3.21 has a huge advantage. It doesn't downshift more to hold gears unless you're going up a really steep hill at full speed in which case the hill could easily be enough to cause the 3.92 to downshift as well.

We're not talking about your dozens of experience with other cars and trucks. We're talking Ram 1500 with the 3.21 and with the 3.92. And all the high MPGs I've seen posted in this forum have been with the 3.21.

I'm not sure why it's so difficult for some to admit the math (theory) and the live numbers (MPG postings) prove the 3.21 is MPG king. It's like you guys won't sleep if somehow the 3.92 doesn't defy physics and do it all better then everybody else.

The 3.92 maximizes pulling power. The 3.21 maximizes MPG. Neither are awful at the other task, just that one does the one task somewhat better than the other. It's really that simple.

I'm going to leave this thread at this point, so happy trails.

 

[JF19Longhorn]

I see what you're trying to say, but I still don't agree with it.

First of all, nobody starts out in 7th gear. And if you ever hopped onto a 10-speed bike as a kid, you know full well that it's easier to start peddling in a lower gear than it is a higher gear. So from a standing stop, the 3.21 truck will burn more fuel just getting up to speed.

Then, once up to speed, it's going to be more difficult for the truck to hold 8th gear with 3.21s, which creates more trans. shifting, which lowers fuel economy and builds heat in the fluid....which was the point of my last post.

So, in the real world, my experience* has been that numerically higher gears are ALWAYS an advantage with the exception of high speed driving on an open interstate. And since I (as well as the average guy) don't spend much of my time at high speed, the 3.92s were a no-brainer for mileage, towing, and general driving. You can lay out numbers on paper, but unfortunately the real world has variables that don't exist on that paper.

*By experience, I mean dozens of midsize and fullsize trucks and SUVs over the last 25 years, with axle ratios from 2.65 to 4.88, many of which I re-geared myself at home for the purpose of experimentation.

I believe this would be true if the engine were being taxed to get the truck moving.. but unloaded, the Hemi isn't close to it's limit, so this just doesn't hold true.

 

[SickSix]

Hi, noob here. Haven't purchased a Ram yet, but looking at a 2020 Limited. In the "build & price" tool on the website, 3 different gear options are listed: 3.21, 3.55, and 3.92. I'm not a gear expert by any means, but it seems like the 3.55 would be the best of both worlds. Is that true, or am I missing something?

 

[silver billet]

Hi, noob here. Haven't purchased a Ram yet, but looking at a 2020 Limited. In the "build & price" tool on the website, 3 different gear options are listed: 3.21, 3.55, and 3.92. I'm not a gear expert by any means, but it seems like the 3.55 would be the best of both worlds. Is that true, or am I missing something?

Different gear ratios for different engine choices, have to make sure you know what engine you're getting as you can't get the 3.55 with the hemi.

 

[SickSix]

Different gear ratios for different engine choices, have to make sure you know what engine you're getting as you can't get the 3.55 with the hemi.

Ah, good call. When you choose 3.55, it changes the engine choice to the V6, didn't notice that before.

 

[Partime43]

So if I'm planning on towing a 25 foot Air Stream across country with my 19 Ram 1500 crew cab 5.7 liter with a axle ratio of 3.21 I,m going to be fine?

 

[Mustangmike66]

So if I'm planning on towing a 25 foot Air Stream across country with my 19 Ram 1500 crew cab 5.7 liter with a axle ratio of 3.21 I,m going to be fine?

How heavy is the trailer and how heavy is the cargo will you be carrying? The payload is often where the 1500 is limited.


Sent from my iPhone using Tapatalk

 

[OldMarine]

every old gear head knows the taller the rear end gears, the better for highway and mpg..the lower the rear end gears, the better out of the hole, (and in this case for pulling). I plan on towing no more than 6000 # so for me the 3:21 works just fine.

 

[RamSco]

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.

I’m not sure I understand. Can you do all that math again assuming I have 35” tires? .

 

[Calsun]

Differences in fuel economy are too small to be of any consequence. What should matter is the towing capacity that is reduced by a thousand pounds or more with the 3.21 gears. Taller gearing helps Ram meet it CAFE target and that is why they along with Ford and Chevy provide the stock gears that they do.

If anyone cares about towing they will get the 3.55 or 3.92 gears and if they really care about fuel economy they will get the V-6 engine. Everything else is noise.

 

[Roddog2220]

i just towed a 5-6k trailer with lift and 35s with 3.92 gears and mine said 11.2 mpg over about 200 miles

 

[Partime43]

How heavy is the trailer and how heavy is the cargo will you be carrying? The payload is often where the 1500 is limited.


Sent from my iPhone using Tapatalk

We are planning on not taking everything and the kitchen sink. trailer wieghts 6200 lbs.

 

[BowDown]

I'm sorry but you are completely incorrect. Unloaded/during commuting, the 3.21 has a huge advantage. It doesn't downshift more to hold gears unless you're going up a really steep hill at full speed in which case the hill could easily be enough to cause the 3.92 to downshift as well.

We're not talking about your dozens of experience with other cars and trucks. We're talking Ram 1500 with the 3.21 and with the 3.92. And all the high MPGs I've seen posted in this forum have been with the 3.21.

I'm not sure why it's so difficult for some to admit the math (theory) and the live numbers (MPG postings) prove the 3.21 is MPG king. It's like you guys won't sleep if somehow the 3.92 doesn't defy physics and do it all better then everybody else.

The 3.92 maximizes pulling power. The 3.21 maximizes MPG. Neither are awful at the other task, just that one does the one task somewhat better than the other. It's really that simple.

I'm going to leave this thread at this point, so happy trails.

I agree. I have a 3:92 geared Ram albeit a Gen 3 but 3:92 none the less. Last spring my wifes XF was in the shop getting some paint work done and the insurance gave me a 2019 Laramie 5.7 3:21 truck for about 10 days. That Truck seemed to accelerate far quicker than my truck (my experience with the 3:92 has been that the advantage disappears after about 30-40mph), granted some of that difference is in the 8 Speed ZF trans vs my 5 speed auto but the truck also DID NOT hunt gears at 70mph on the highway. Around town, that truck got 17-19 driving the same way I drive my truck which gets 13mpg. On the highway, the 2019 was getting 21-22mpg to my 15-16. I was already looking at a 3:21 geared truck and that week or so test drive only re-enforced by belief.
I'm looking at a 2020 Limited 4x2 now with 3:21, waiting to see what Covid19 will do to the prices

 

[BowDown]

Differences in fuel economy are too small to be of any consequence. What should matter is the towing capacity that is reduced by a thousand pounds or more with the 3.21 gears. Taller gearing helps Ram meet it CAFE target and that is why they along with Ford and Chevy provide the stock gears that they do.

If anyone cares about towing they will get the 3.55 or 3.92 gears and if they really care about fuel economy they will get the V-6 engine. Everything else is noise.

The difference in fuel economy would be about 22% as thats the difference between the two gear sets . My truck with 3:92 gears is turning 1900 RPM @ 65-67 mph. 22% less gear/rpm would have me at 1482 RPM (ca;l it 1500rpm). My fuel economy at 1900rpm is about 17 mpg on flat highways (Texas), at 1500 rpm, my economy jumps to 19-20 but that's about 52-55 mph. Over a Dallas to Houston drive, that's a 70 per tank difference so its not small.

 

[RamSco]

When I bought my truck and still for now, I live in the Mountain West where towing up steep grades is a fact of life. That, combined with the fact that I put 35's on my pickup, made it a no-brainer to get 3.92's. I'm not unhappy about the fuel economy as it's about the same as my '07 2500 6.7 Cummins was and gasoline is still less $ per gallon than diesel. I'm sure if I lived in Texas I would have 3.21's. Nobody is right and nobody is wrong in the choice they made for their own truck or lifestyle. Part of my problem is the speed limits on dessert freeways these days is usually 80 MPH and I can't not drive at least the speed limit

 

[zchamber]

Just wanted to add another perspective. I got 3.92 for my Limited ED with ORP not to tow or anything as payload is barely 1050lbs. I wanted to put larger tires ie 34 inch vs 32 inch which would have brought my axle ratio down to almost 3.65 and have enough kick. I am turning about 200 rpm less at 65 and 75 mph now than before. 3.21 would have made it very weak gearing. I noticed on the same loop a gain of almost 1 mpg but had to go auto 4 instead of 2wd so will have to do another run in dry weather to get a better picture. One more thing to add that I made sure the 34 inch tire is about the same weight and contact patch (275) as the OE Falken WP so effectively seeing more of a gearing effect. Perhaps 3.55 would have worked about the same with even better mileage but was not available with eco diesel.