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

[VernDiesel]

Willow makes complete sense to me for someone who’s truck is primarily recreational. I have an ole Buick Grand National. I keep her wicked up to hot rod around in. Under boost she drinks it like any big block hot rod. I don’t care because I’m only in it when I have time & money for. It’s about going fast not fuel economy.

Now my truck is my business. I’m paid by the mile & fuel towing travel trailers is my single biggest expense. It can be the difference between making money & treading water. Nobody does this business with a gas truck because eventually they would be in the bread line.

 

[JJRamTX]

Thanks for putting this table and information together! With 8 gears there is very good overlap and as stated prior hopefully the engineers got the software shift profile correct. Since there have been a couple transmission shift profile software flashes they are still tweeking it a bit....
With the addition of eTorque I am very happy with my towing ability now and this truck is amazeballs compared to my last Hemi 5.7 3.92 that was rated at 345HP and 375 Ft Lbs. Now I get the extra 50HP and 410 Ft Lbs of Torque constantly which makes a huge difference getting 7 tons+ rolling as well as actually gives the ability to pass on a 2 lane road while towing.

 

[jdmartin]

It shouldn't matter why someone is or isn't concerned about mpg... My employer pays for my fuel, but it still concerns me that I get the lowest range mpg of any other I see reported.

Why? THAT to me is something I would consider goofy. Would you think that of anything else?

"My employer pays me for my work boots. Why do my boots wear out faster than Tony's boots?"
"My employer pays me for my tools. Why does my Philips screwdriver lose its edge faster than John's?"
"My employer pays me for lunch. Why do I still feel hungry when Bubba's full on the same money?"

Other than an obvious defect, almost all differences can be traced back to the ultimate variable: you.

Maybe your work boots wear out faster because you're heavier, or your gait is more pronounced.
Maybe your screwdriver loses its edge faster because you don't switch to the exact size as often.
Maybe you're still hungry because you buy a hot dog and fries instead of a salad, fruit, and hefty sandwich.

I *can* get fantastic fuel economy if I want to. I've got well over a million miles on my *** and probably 20 regular commuter cars/work trucks (not counting the fun stuff), and I've always been able to exceed the EPA estimates on every vehicle, even though many of them were bought old and already worn. How is this possible? Because the ultimate variable is me.

I've shown the numbers here before and to me it doesn't make sense to worry about something that A) you can likely improve if you really want to work at it, and B) doesn't make hardly any difference in money if you don't worry about it.

Finally, it doesn't bother me if someone else wants to worry about it - have at it! I just don't make any sense of it. Life has enough real worries - kids/parents/wife gets sick, people close to you die, your animals get sick, you lose a job and run into financial trouble, you get sick. Those are real worries. Fretting about gas mileage will just shorten your lifespan by adding needless stress for an extra mile or two per gallon.

So do like Nissan and enjoy the ride

 

[DesertRebel]

Why? THAT to me is something I would consider goofy. Would you think that of anything else?

"My employer pays me for my work boots. Why do my boots wear out faster than Tony's boots?"
"My employer pays me for my tools. Why does my Philips screwdriver lose its edge faster than John's?"
"My employer pays me for lunch. Why do I still feel hungry when Bubba's full on the same money?"

Other than an obvious defect, almost all differences can be traced back to the ultimate variable: you.

Maybe your work boots wear out faster because you're heavier, or your gait is more pronounced.
Maybe your screwdriver loses its edge faster because you don't switch to the exact size as often.
Maybe you're still hungry because you buy a hot dog and fries instead of a salad, fruit, and hefty sandwich.

I *can* get fantastic fuel economy if I want to. I've got well over a million miles on my *** and probably 20 regular commuter cars/work trucks (not counting the fun stuff), and I've always been able to exceed the EPA estimates on every vehicle, even though many of them were bought old and already worn. How is this possible? Because the ultimate variable is me.

I've shown the numbers here before and to me it doesn't make sense to worry about something that A) you can likely improve if you really want to work at it, and B) doesn't make hardly any difference in money if you don't worry about it.

Finally, it doesn't bother me if someone else wants to worry about it - have at it! I just don't make any sense of it. Life has enough real worries - kids/parents/wife gets sick, people close to you die, your animals get sick, you lose a job and run into financial trouble, you get sick. Those are real worries. Fretting about gas mileage will just shorten your lifespan by adding needless stress for an extra mile or two per gallon.

So do like Nissan and enjoy the ride

Well my friend, you're wrong.

This truck has got mid 13s for 3K miles... no variance in MPG regardless of driving styles.

The reason I mentioned my employer paying for fuel is - many times people imply truck purchases are made without regard to fuel economy... as if someone thinks they are going to get Prius mpg from a hemi... my concern isn't that a truck get less mileage (this is 4th truck in 13 yrs, I'm fully aware of mpg), my concern is that most dont get this low of mpg... not others with same truck and not any other trucks I've had.

Having that been siad; I'm not "fretting it" - it's not that big of a deal to me... what is a bIg deal to me is others telling people they shouldn't worry about their fuel mileage. If MPG sux and people mention it, deal with it. It's a fact.

 

[jdmartin]

Well my friend, you're wrong.

This truck has got mid 13s for 3K miles... no variance in MPG regardless of driving styles.

The reason I mentioned my employer paying for fuel is - many times people imply truck purchases are made without regard to fuel economy... as if someone thinks they are going to get Prius mpg from a hemi... my concern isn't that a truck get less mileage (this is 4th truck in 13 yrs, I'm fully aware of mpg), my concern is that most dont get this low of mpg... not others with same truck and not any other trucks I've had.

Having that been siad; I'm not "fretting it" - it's not that big of a deal to me... what is a bIg deal to me is others telling people they shouldn't worry about their fuel mileage. If MPG sux and people mention it, deal with it. It's a fact.

Hmmm.....I must have magic pixie dust in my truck then. I am in the mountains, have a 4WD crew cab with the 1" lift, 5.7 no e-torque, and the few times I've checked to see what I'm getting it's in the high teens. I got almost 20 MPG on my recent trip to FL and back. I guess I just got one of those non-faulty trucks that came off the line, or maybe my truck got some extra MPGs from some other guy's trucks

 

[Willwork4truck]

Willow makes complete sense to me for someone who’s truck is primarily recreational. I have an ole Buick Grand National. I keep her wicked up to hot rod around in. Under boost she drinks it like any big block hot rod. I don’t care because I’m only in it when I have time & money for. It’s about going fast not fuel economy.

Now my truck is my business. I’m paid by the mile & fuel towing travel trailers is my single biggest expense. It can be the difference between making money & treading water. Nobody does this business with a gas truck because eventually they would be in the bread line.

In that case (business) then yes, cost per mile (cpm) to operate is a big factor for you, and fuel is often the 2nd biggest next to the payment.

 

[DesertRebel]

Hmmm.....I must have magic pixie dust in my truck then. I am in the mountains, have a 4WD crew cab with the 1" lift, 5.7 no e-torque, and the few times I've checked to see what I'm getting it's in the high teens. I got almost 20 MPG on my recent trip to FL and back. I guess I just got one of those non-faulty trucks that came off the line, or maybe my truck got some extra MPGs from some other guy's trucks

Well at least you see my point now.

Your truck is not unusual mine is. And no, it's not the driving.

So when some of us mention crappy MPG and others say deal with it - that's what I'm referring to... not the guy complaining about getting what most trucks get, the few of us that are getting far below what is typical.

Knee jerk, clueless comments about lead foot, etc - are silly. But it's the internet, and everyone thinks they are experts and have clairvoyance - so I'm sure there will be some retort (full of ignorance no doubt).

 

[Klute]

I assume both ratios perform the same going downhill?

 

[derp]

Why? THAT to me is something I would consider goofy. Would you think that of anything else?

"My employer pays me for my work boots. Why do my boots wear out faster than Tony's boots?"
"

"My employer pays for my fuel, why am I ****ting out so much more crap through the tailpipe and having to fill up more often than Tony?"

 

[Phoebe]

That's great! I appreciate that. Thank you for such a helpful explanation.

 

[tcm4950]

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.

Thank you
A most excellent read

 

[Timeless]

Hmmm.....I must have magic pixie dust in my truck then. I am in the mountains, have a 4WD crew cab with the 1" lift, 5.7 no e-torque, and the few times I've checked to see what I'm getting it's in the high teens. I got almost 20 MPG on my recent trip to FL and back. I guess I just got one of those non-faulty trucks that came off the line, or maybe my truck got some extra MPGs from some other guy's trucks

Could you spare some pixie dust? Mine is at 13 also...mainly city though I admit.

Well at least you see my point now.

Your truck is not unusual mine is. And no, it's not the driving.

So when some of us mention crappy MPG and others say deal with it - that's what I'm referring to... not the guy complaining about getting what most trucks get, the few of us that are getting far below what is typical.

Knee jerk, clueless comments about lead foot, etc - are silly. But it's the internet, and everyone thinks they are experts and have clairvoyance - so I'm sure there will be some retort (full of ignorance no doubt).

Not sure yours is unusual as I get 13 also.

 

[DesertRebel]

Could you spare some pixie dust? Mine is at 13 also...mainly city though I admit.



Not sure yours is unusual as I get 13 also.

I didn't say I was the only one - in fact, I've mentioned the opposite (see previous posts)... but our MPG is definitely not the norm (as indicated all over this forum)... therefore "unusual".

The usual is 15-17 and in many cases there are unusually high numbers 20+

I'm getting low to mid 12s now - I dont even see 13 anymore... 12-13 is absolutely unusual (not to be confused with no other truck is that low).

 

[WXman]

Aside from the obvious which is that 3.92s get the truck out of the hole faster, accelerate 0-60 harder, and tow a trailer better, I found after driving mine for a little over a week now that they're actually a benefit to me on the highway.

My commute daily is mainly on 4-lane limited access highways with a speed limit of 55 MPH. Most traffic is running between 55-65 MPH. With my cruise control set to 58 MPH, my truck cruises in 8th gear at 1,600 RPM. If I had the 3.21 axle gears, the RPM would fall low enough that the truck would frequently downshift to 7th. There would be a lot more downshift/upshift/downshift/upshift similar to what the gas engines do. There is simply no way that would result in higher MPGs.

By staying in 8th gear all the time, I'm actually saving fuel on the highway.

I still say there is no situation outside of running 80 MPH on the interstate that the 3.92 gears are not a benefit.

 

[silver billet]

Aside from the obvious which is that 3.92s get the truck out of the hole faster, accelerate 0-60 harder, and tow a trailer better, I found after driving mine for a little over a week now that they're actually a benefit to me on the highway.

My commute daily is mainly on 4-lane limited access highways with a speed limit of 55 MPH. Most traffic is running between 55-65 MPH. With my cruise control set to 58 MPH, my truck cruises in 8th gear at 1,600 RPM. If I had the 3.21 axle gears, the RPM would fall low enough that the truck would frequently downshift to 7th. There would be a lot more downshift/upshift/downshift/upshift similar to what the gas engines do. There is simply no way that would result in higher MPGs.

By staying in 8th gear all the time, I'm actually saving fuel on the highway.

I still say there is no situation outside of running 80 MPH on the interstate that the 3.92 gears are not a benefit.

This is incorrect; when you're cruising in 8th with a 3.92, for the same RPM I'm cruising in 7th with a 3.21. So to to prevent downshifting I can just run in 7 while you're running in 8th, and voila, no downshifting in my truck. However, I can STILL upshift another gear yet, which you can't.

For my usage, my truck spends more time in 8th than any other gear, by a HUGE margin. I do mostly rural and highway driving of course.

 

[Dustin]

Thanks for this explanation.

 

[MrCuervoRacing]

.........folks feel they need to defend their purchases to the death, regardless of facts.


Cheers,

Best thing ive seen said in quite a while... i will have to borrow that .

 

[MrCuervoRacing]

This is incorrect; when you're cruising in 8th with a 3.92, for the same RPM I'm cruising in 7th with a 3.21. So to to prevent downshifting I can just run in 7 while you're running in 8th, and voila, no downshifting in my truck. However, I can STILL upshift another gear yet, which you can't.

For my usage, my truck spends more time in 8th than any other gear, by a HUGE margin. I do mostly rural and highway driving of course.

unloaded..... 8th gear in Eco on 3.92 vs 8th gear on 3.21 non eco due to load. who wins.

 

[silver billet]

unloaded..... 8th gear in Eco on 3.92 vs 8th gear on 3.21 non eco due to load. who wins.

I never use ECO anymore; I don't like the sound and it doesn't do much in my usage. But my mistake, RAM engineers must have given us the 3.21 for no reason other than to laugh while we have fun discussing it.

See below for a 3 hour trip I did. This was still when I was using ECO, I've done the same trip later on with it disabled and got 9.8. When you can post a better MPG (converted, 9.7 L/100Km = 24 mpg) I'll rethink my viewpoint, until then, the 3.21 is the MPG king. I'm not saying you picked the wrong gear for your usage, or that you made a mistake. I'm saying it's a FACT that the 3.21 gets better MPG. You should re-read your second last post in this thread.

 

[silver billet]

unloaded..... 8th gear in Eco on 3.92 vs 8th gear on 3.21 non eco due to load. who wins.

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.