I don't fully agree with this guy,as the stock shift points are 5600 rpm,not the 6,000 he's using,but still a decent article on gearing

[Wild one]

• 
  #1

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.

 

[HEMIMANN]

Oh, hell no! I value longevity of the powertrain over saving $1,000 in gas over the shortened engine life, whatever amount those are.

Since I got 6 speed and found they split 5th and 6th O.D. ratios around the former single 5th O.D. ratio, I only tow in 4th, 1:1 with 3.73 axle ratio.

It shifts between 4th and 3rd on hills. Most of these 4th gens came with 3:73 ratio axles unless you wanted to order higher and wait.

 

[Brandon-w]

I'll stick with the 3.92 I still get wicked economy and tows great I don't think I'd ever be happy with 3.42

 

[Ken226]

• 
  #1

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.

So, the 3.55 kinda splits the difference?

 

[Wild one]

So, the 3.55 kinda splits the difference?

They only offered the 5.7 /8 speed combo with 3.55's in MY 13's after that they dropped the 3.55's with the Hemi/8 speed. Personally i think that was a mistake,as the 3.55's would benefit alot of guys,who don't really need 3.92's,but don't want 3.21's.

 

[crash68]

but wait there's more on the Gen4 trucks there two different tire diameters that can also effect a vehicle towing prowellness. The difference is a little over 4%
Gen5 trucks wheel combos are pretty much all the same diameter (TRX and Rebel excluded).

 

[crash68]

I don't think I'd ever be happy with 3.42

Unless there's a Cummins under the hood, it has enough torque to twist through the taller ratio and the less than appealing gear stagger of the 68RFE

 

[CanuckRam1313]

They only offered the 5.7 /8 speed combo with 3.55's in MY 13's after that they dropped the 3.55's with the Hemi/8 speed. Personally i think that was a mistake,as the 3.55's would benefit alot of guys,who don't really need 3.92's,but don't want 3.21's.

3.92's all the way, Baby!!!!!!

 

[HEMIMANN]

Back in the day 3.21 was unheard of except for diesel.

 

[Wild one]

3.92's all the way, Baby!!!!!!

View attachment 562779

I spend to much time on the QE2 between Calgary and Edmonton to be a fan of 3.92's,in fact i hated the 3.55's compared to the 3.21's when i swapped gears. At 80+ mph the 3.92's leave alot to be desired,lol.I've had a few guys follow me up to the track in Edmonton,and i don't waste time getting there,and by the time we hit Red Deer they're wanting to stop to let their trucks cool down,as their oil temps are well above 220 after 90 miles of 85+ mph,lol
Even switching from 3.21's to 3.55's increased my oil temps more then i liked,and also knocked a good 6/7 mph off the trucks top end,and playing with the local LX group,i've had to prove several times the truck would easily keep up with a Challenger/Charger on the highway
I also didn't gain squat at the track with the 3.55's

 

[ramffml]

Back in the day 3.21 was unheard of except for diesel.

Yes but the rear axle can never be looked at in isolation, what is the first gear ratio in the transmission?

The ZF 8 speed with a 3.21 has a shorter final gear ratio in first, than the toyota tundra 6 speed with a 4.30. That's because the 1'st gear of the 8 speed zf is 5.0 vs the tundra's 3.33

first gear in ZF = 5 * 3.21 = 16.05
first gear in tundra = 3.33 * 4.3 = 14.32

In other words, the 3.21 kicks the tundra's a$$ not only in off the line performance, but also in overdrive/freeway while having 2 more gears to play with on top of it all.

Gotta love high gear count transmissions, they have made the rear axle ratio pretty much irrelevant.

 

[Docwagon1776]

I'm more likely to be crawling in a field than screaming on the interstate in a truck, so numerically higher gears are better for me in general.

One nice thing about my old '87 granny gear 4 speed Ram, in 4-lo and 1st you could take a fence down by yourself. The truck would idle along slowly enough you could work the post out, let the truck catch up, drop it in the bed, walk to the next post, repeat. Every now and then it would need some steering input, but for the most part is just followed you along like an old dog.

 

[Wild one]

Yes but the rear axle can never be looked at in isolation, what is the first gear ratio in the transmission?

The ZF 8 speed with a 3.21 has a shorter final gear ratio in first, than the toyota tundra 6 speed with a 4.30. That's because the 1'st gear of the 8 speed zf is 5.0 vs the tundra's 3.33

first gear in ZF = 5 * 3.21 = 16.05
first gear in tundra = 3.33 * 4.3 = 14.32

In other words, the 3.21 kicks the tundra's a$$ not only in off the line performance, but also in overdrive/freeway while having 2 more gears to play with on top of it all.

Gotta love high gear count transmissions, they have made the rear axle ratio pretty much irrelevant.

The 8HP75 is 5:00:1 in first,the 8HP70 is 4.71:1 in first gear,not that it really changes things much,but they are geared slightly differant

 

[Wild one]

I'm more likely to be crawling in a field than screaming on the interstate in a truck, so numerically higher gears are better for me in general.

One nice thing about my old '87 granny gear 4 speed Ram, in 4-lo and 1st you could take a fence down by yourself. The truck would idle along slowly enough you could work the post out, let the truck catch up, drop it in the bed, walk to the next post, repeat. Every now and then it would need some steering input, but for the most part is just followed you along like an old dog.

Granny geared 4 speed 4X4's were Tractor geared, in 4lo they'd crawl up the side of a mountain at idle,lol

 

[HEMIMANN]

Yes but the rear axle can never be looked at in isolation, what is the first gear ratio in the transmission?

The ZF 8 speed with a 3.21 has a shorter final gear ratio in first, than the toyota tundra 6 speed with a 4.30. That's because the 1'st gear of the 8 speed zf is 5.0 vs the tundra's 3.33

first gear in ZF = 5 * 3.21 = 16.05
first gear in tundra = 3.33 * 4.3 = 14.32

In other words, the 3.21 kicks the tundra's a$$ not only in off the line performance, but also in overdrive/freeway while having 2 more gears to play with on top of it all.

Gotta love high gear count transmissions, they have made the rear axle ratio pretty much irrelevant.

Waiting for the hydrostatic truck tranny. THAT'LL be efficient! like, 60%, maybe? lol

CVT done ancient

OTOH, high gear count trannys are becoming problematic in complexity and shifting. More parts, more software, less quality and reliability. Just like anything. Maybe they'll go with wheelmotors and generators like haul trucks and locomotives? All it takes is lots more money.

 

[ramffml]

Waiting for the hydrostatic truck tranny. THAT'LL be efficient! like, 60%, maybe? lol

CVT done ancient

OTOH, high gear count trannys are becoming problematic in complexity and shifting. More parts, more software, less quality and reliability. Just like anything. Maybe they'll go with wheelmotors and generators like haul trucks and locomotives? All it takes is lots more money.

The ZF has been rock solid in every FCA application as far as I know. Join the party, splurge a little, you have to get yourself a nice 2019+ HD

 

[HEMIMANN]

The ZF has been rock solid in every FCA application as far as I know. Join the party, splurge a little, you have to get yourself a nice 2019+ HD

LOL

No, really LOL

Carlos wrecked them. Yes, I love the ZF. No, I hate the German insistence on solely professional service. Strange culture. I REALLY wanted to put a new valve body in my 66RFE, but didn't know enough when I got it to upgrade chronologically correctly. Then I'd have a nearly perfect truck. Best ride I've ever had, most comfy, plenty of power, capacity, etc. etc.

I was just out today hauling farm store supplies. It rides soooo nice.

 

[Brandon-w]

Unless there's a Cummins under the hood, it has enough torque to twist through the taller ratio and the less than appealing gear stagger of the 68RFE

Now You're talking my language! Add more power it solves any gearing issues

 

[04fxdwgi]

Granny geared 4 speed 4X4's were Tractor geared, in 4lo they'd crawl up the side of a mountain at idle,lol

Plus, the old 4 speed PTO was engaged in the xfer case after the final drive ratio (had to be in gear with the xfer case in N to run PTO). So one could run the PTO in 4 different speeds. Worked great in our off road recovery tow trucks when doing long line recoveries with 500' cables or using 1 or more snatch blocks..

GM and Ford, not so much, as the PTO came off the xmission..

 

[Brandon-w]

Plus, the old 4 speed PTO was engaged in the xfer case after the final drive ratio (had to be in gear with the xfer case in N to run PTO). So one could run the PTO in 4 different speeds. Worked great in our off road recovery tow trucks when doing long line recoveries with 500' cables or using 1 or more snatch blocks..

GM and Ford, not so much, as the PTO came off the xmission..

My cummins has two PTO on the transmission and one off the t case blew me away. Just finished rebuilding it, if she gets too hot I'll run some lines off that billet plate to a cooler