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

[SnowBlaZR2]

Yeah my 00 silverado always looked like baby poop, will never buy blue again

That's what garages and weekly maintenance washes are for.

 

[SnowBlaZR2]

I might have missed this in this very good, but pages long, thread but what type of fuel savings are we talking about in terms of 3.21 over 3.92? I would never tow anything over 4,000 lbs., because I'm just not that confident in my ability and would hire someone. More to the point, my boat weighs about 3,500 pounds so fuel is a bigger factor.

Depends how you drive your truck. If you spend nearly all of your time on the Interstates doing 70+ mph, there's a chance you could see some negligible savings.

Your right foot has more to do with your fuel economy than your gearing.

Go with the 3.92s or you'll always wish you had and will spend your days here making up stories about improved fuel economy.

 

[theblet]

I might have missed this in this very good, but pages long, thread but what type of fuel savings are we talking about in terms of 3.21 over 3.92? I would never tow anything over 4,000 lbs., because I'm just not that confident in my ability and would hire someone. More to the point, my boat weighs about 3,500 pounds so fuel is a bigger factor.

I’d say 3 to 4 mpg stock. Different people show different fuel mileage. How far and how often will you be pulling your boat? 3.21s will pull it fine, but it’ll be easier to pull, and slip launch/recover with 3.92s.

 

[Idahoktm]

I’d say 3 to 4 mpg stock. Different people show different fuel mileage. How far and how often will you be pulling your boat? 3.21s will pull it fine, but it’ll be easier to pull, and slip launch/recover with 3.92s.

You wouldn't see numbers like that, even if you drove 80 mph 100% of the time.

 

[HSKR R/T]

I might have missed this in this very good, but pages long, thread but what type of fuel savings are we talking about in terms of 3.21 over 3.92? I would never tow anything over 4,000 lbs., because I'm just not that confident in my ability and would hire someone. More to the point, my boat weighs about 3,500 pounds so fuel is a bigger factor.

To many variables to give an accurate answer. If you do mostly city street driving under 50mph, you might not notice any difference, or possibly even slight advantage to 3.92. mostly freeway driving on flat ground, advantage 3.21. Add in hills, headwinds, temperature fluctuations........it could be a wash. Get what makes you feel better about your choice and don't look back. It won't be enough difference for any significant money savings long term.

 

[parnine]

I’d say 3 to 4 mpg stock. Different people show different fuel mileage. How far and how often will you be pulling your boat? 3.21s will pull it fine, but it’ll be easier to pull, and slip launch/recover with 3.92s.

No way you see that (3-4 mpg) difference, EVERYTHING else being equal, between 3.21 and 3.92. Maybe get 1 mpg on pure highway, otherwise it will be negligible. (folks post 1-2mpg)

 

[oldsouth]

For some of us, it is more about acceleration and power that it is about MPG or towing ability.

 

[Ramjack]

I’d say 3 to 4 mpg stock. Different people show different fuel mileage. How far and how often will you be pulling your boat? 3.21s will pull it fine, but it’ll be easier to pull, and slip launch/recover with 3.92s.

No way. All else being equal, I'd say 1-2mpg tops.

 

[Ramjack]

If you do mostly city street driving under 50mph, you might not notice any difference, or possibly even slight advantage to 3.92. mostly freeway driving on flat ground, advantage 3.21. Add in hills, headwinds, temperature fluctuations........it could be a wash.

3.21s have a fuel efficiency advantage (albeit small) under virtually all scenarios. 3.92s have the advantage in virtually everything except fuel efficiency.

Get what makes you feel better about your choice and don't look back. It won't be enough difference for any significant money savings long term.

I agree with this. If my dealer would've had 3.92 on the lot, I would've gotten it. But since I don't tow (at all), I have no regrets.

 

[HSKR R/T]

3.21s have a fuel efficiency advantage (albeit small) under virtually all scenarios. 3.92s have the advantage in virtually everything except fuel efficiency.

Not true. If you could theoretically have the 3.21s in the same gear as the 3.92 under every circumstance, that would be true. But that will never be the case. You can quote final drive ratios numbers till you are blue in the face, but that assumes that both hear sets will be in the same gear at all times.

 

[Ramjack]

Not true. If you could theoretically have the 3.21s in the same gear as the 3.92 under every circumstance, that would be true. But that will never be the case. You can quote final drive ratios numbers till you are blue in the face, but that assumes that both hear sets will be in the same gear at all times.

Ok, I will grant you that it's plausible if the scenario results in the the 3.92 truck spending more time at lower rpm than the 3.21 truck. Not sure that's going to happen very often.

 

[Ramit392]

Great write up and great read! One thing to keep in mind folks who buy a truck should really not be looking at MPG to start with, since the The trucks are not really for that purpose. its kind of like the guy who ask me other day if I was going to keep driving the Hellcat when fuel hits 8 to 10 bucks a gallon? I told him if MPG and cost of fuel was an issue I would have never bought a Hellcat! I cannot imagine why FCA decided to put a 3.21 Gear in a truck to begin with? It really should have been 3.55 or the 3.92 but I suppose the 3.21 was because of the 8 speed which is not really needed for a 3.92 as your write up pretty well points out.

 

[m.hamner]

I might have missed this in this very good, but pages long, thread but what type of fuel savings are we talking about in terms of 3.21 over 3.92? I would never tow anything over 4,000 lbs., because I'm just not that confident in my ability and would hire someone. More to the point, my boat weighs about 3,500 pounds so fuel is a bigger factor.

a couple gallons at most. 2mpg isn't a lot, but it factors as a bigger differential at the higher price points for gas. I did some math, and it might be the difference of 790 dollars per year with gas being 4 bucks based on how much I drive per year and the way I drive which is mainly 90% hwy at over 70mph. I don't tow except for the once off and it's pretty low weight, so I don't need the extra torque from the gearing.

 

[RamDutyI]

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 have the 3.92 and drive flat midwestern roads. No engineer am I, yet I still notice I can crest slight inclines while still in eco mode with cylinder deactivation. Even on four cylinders active, seems plenty of torque for a pleasurable drive. As for fuel mileage, drive it light and slow a bit, and you'll have surprising gas mileage. Drove 100 miles on an interstate at 60 or 65mph. Nearly got 26mpg, the mileage I got with old EFI VW's. Doing 40-45mph on back roads, and it's over 30mpg.

Ya, retirement and not needing to get anywhere on time is kinda fun... Great article. Got any engineer tips on getting more mpg? Tire pressure has always been key for me.

 

[Mountain Whiskey]

I have the 3.92 and drive flat midwestern roads. No engineer am I, yet I still notice I can crest slight inclines while still in eco mode with cylinder deactivation. Even on four cylinders active, seems plenty of torque for a pleasurable drive. As for fuel mileage, drive it light and slow a bit, and you'll have surprising gas mileage. Drove 100 miles on an interstate at 60 or 65mph. Nearly got 26mpg, the mileage I got with old EFI VW's. Doing 40-45mph on back roads, and it's over 30mpg.

Ya, retirement and not needing to get anywhere on time is kinda fun... Great article. Got any engineer tips on getting more mpg? Tire pressure has always been key for me.

Trust me, I correct engineer's work all day every day. If you want to figure out the most complicated way to not make something work, get one involved.

 

[theblet]

I have the 3.92 and drive flat midwestern roads. No engineer am I, yet I still notice I can crest slight inclines while still in eco mode with cylinder deactivation. Even on four cylinders active, seems plenty of torque for a pleasurable drive. As for fuel mileage, drive it light and slow a bit, and you'll have surprising gas mileage. Drove 100 miles on an interstate at 60 or 65mph. Nearly got 26mpg, the mileage I got with old EFI VW's. Doing 40-45mph on back roads, and it's over 30mpg.

Ya, retirement and not needing to get anywhere on time is kinda fun... Great article. Got any engineer tips on getting more mpg? Tire pressure has always been key for me.

Sounds about right. Only other thing I do is set the cruise, not use remote start, and minimize idling

 

[mike146]

All I can say is that somebody was looking out for me, I got myself a rebel and didn't have to even consider any of this nonsense.

But of course I would have went 3.92 regardless. Be prepared.

 

[theblet]

All I can say is that somebody was looking out for me, I got myself a rebel and didn't have to even consider any of this nonsense.

But of course I would have went 3.92 regardless. Be prepared.

Did you get 3.21 or 3.92 gears?

 

[TNRamGuy]

Did you get 3.21 or 3.92 gears?

Rebel only has 3.92

 

[egnarp]

I owned a 2019 with 3.21s, and now I own a 2021 with 3.92s. Everything else on the trucks are virtually the same. I'm getting about 1 mpg less with the 3.92s. I like the way the 3.92 pulls my boat, and I don't miss the 3.21s always shifting down to 7th gear on mild highway inclines.