3.21. vs 3.92 gear

[Electrical]

I have the 3.21 gears, pulled my boat for the first time the last two days. Fairly flat land. Boat is a 20' aluminum bass boat with 115 optimax on it, not sure of the gross weight of the rig.
Could barely tell it was back there, on the interstate running 70-75 mph it showed between 10-12 mpg. Once off the interstate and on the country back highways running 60 mph, it did 14-16 mpg. Had a very comfortable ride, and no trouble at all handling the boat. Was a big difference from pulling it with my 2013 Ford F-150 (3:55 Gears), it handled it fine but you knew you had it back there, especially from dead stop. I'm glad I went with the 3:21 gears, the boat is primarily what I pull so I think I'm going to get along with it just fine.

14-16 mpg towing? Very good number.

Was your F150 ecoBoost? What mpg towing?

 

[JDBob35a]

14-16 mpg towing? Very good number.

Was your F150 ecoBoost? What mpg towing?

My F150 had the 5.0 V8. It got it's best mileage after 50,000 miles, when I traded for the new RAM it had 93,000. Pulling the same boat it would avg around 10 on interstate, and 13 or so on highways. (87 Octane)
The new Ram was doing the numbers I posted earlier over the weekend with mid 50s ambient temps and burning 89 octane.

 

[2wd]

Is the 3.92 better for towing? Of course. But is the 3.21 bad at towing? Not at all. It is pretty underrated in fact.

If you compare to a 2011-2015 F150 5.0 Max Tow w/3.73 rear end (highest offered), the 3.21 w/ZF8 has a mechanical advantage in 5 of the first 6 gears. Those F150's pulled great and were rated to tow 9000-10,300 lbs. This doesn't even account for the slight HP/Torque advantage the 5.7 has over the Ford 5.0.

It's true the 3.92 will pull better than the 3.21, but to say the 3.21 is horrible at towing (or you can't tow in the mountains within your weight limits) is nonsense. This truck with 3.21's is more capable than Ford's 6 Speed 5.0 Max Tow setup, which towed great.

 

[Ramit392]

IMO opinion Ram could really move the 1/2 ton up even more with 6.4 and a 3.55 gear with more payload,and Pull caps. and with MPG between the 3.92 and 3.21 gear.

 

[Chris]

IMO opinion Ram could really move the 1/2 ton up even more with 6.4 and a 3.55 gear with more payload,and Pull caps. and with MPG between the 3.92 and 3.21 gear.

The 3.92 is to reduce stress on the transmission and drivetrain components far more then the engine; if you use the same transmission, a 3.55 gear will lower your towing rating even with a 6.4 engine.

The main reason the 3.55 takes a tow rating hit on the v6 is that it is a much lower rated transmission. In reality, that v6 can pull the same as the v8 if it had a 3.92 and the upgraded tranny, but it would just be much more speed limited on hills (probably struggle to maintain 45mph on the Ike). Truck tow ratings are based on safety of control and stopping, which barely involves the motor; that's why you see 3/4 ton trucks rated to tow 30,000 Lbs, but if you put the full load in, they can only crawl up the Ike well below 60mph. That's also why they can use the same 5.7 engine in those trucks.

 

[Lateralus]

that's why you see 3/4 ton trucks rated to tow 30,000 Lbs, but if you put the full load in, they can only crawl up the Ike well below 60mph. That's also why they can use the same 5.7 engine in those trucks.

Back away from the keyboard... fake news

 

[Zeronet]

The 3.92 is to reduce stress on the transmission and drivetrain components far more then the engine; if you use the same transmission, a 3.55 gear will lower your towing rating even with a 6.4 engine.

The main reason the 3.55 takes a tow rating hit on the v6 is that it is a much lower rated transmission. In reality, that v6 can pull the same as the v8 if it had a 3.92 and the upgraded tranny, but it would just be much more speed limited on hills (probably struggle to maintain 45mph on the Ike). Truck tow ratings are based on safety of control and stopping, which barely involves the motor; that's why you see 3/4 ton trucks rated to tow 30,000 Lbs, but if you put the full load in, they can only crawl up the Ike well below 60mph. That's also why they can use the same 5.7 engine in those trucks.

True, the higher numerical gear ratios get the higher tow ratings to limit stress on the transmission and differential. Note how the max-tow differential increases the tow rating. The new 8HP75 transmission in the Hemi is rated for 553 lb/ft of torque. Likely they upgraded the transmission for the 2019 just so it could handle that higher max-tow rating in combination with the max-tow differential?

 

[NDanecker]

The 3.92 is to reduce stress on the transmission and drivetrain components far more then the engine; if you use the same transmission, a 3.55 gear will lower your towing rating even with a 6.4 engine.

The main reason the 3.55 takes a tow rating hit on the v6 is that it is a much lower rated transmission. In reality, that v6 can pull the same as the v8 if it had a 3.92 and the upgraded tranny, but it would just be much more speed limited on hills (probably struggle to maintain 45mph on the Ike). Truck tow ratings are based on safety of control and stopping, which barely involves the motor; that's why you see 3/4 ton trucks rated to tow 30,000 Lbs, but if you put the full load in, they can only crawl up the Ike well below 60mph. That's also why they can use the same 5.7 engine in those trucks.

I'd like to respectfully state a number of your comments are incorrect.

1 - 3.21 or 3.92 gears will not effect the amount of maximum stress in the transmission. You cannot have more stress / torque than what the motor generates no matter how much you tow. The mechanical advantage of changing the final gear ratio can add more stress to the axle splines (as a result of the mechanical advantage the gear set yields) but those are not the weakest link. The tires will break free long before you come close to the yield strength of the axle shafts in the axle.

2 - Vehicle tow ratings are based on what the vehicle is capable to tow while meeting certain acceleration and stopping requirements. Unlike your statement '...which barely involves the motor...' the tow ratings do heavily involve the motor in what many manufacturers have recently accepted as outlined in SAE J2807 standards. HOWEVER, these tow ratings are for marketing, warranty claims, and shopping purposes between vehicle brands only. No law states you cannot exceed the 'recommended' tow ratings set forth by the manufacturer. For example, many commercial haulers (class 3, 4 and 5 trucks) routinely tow well beyond their mfg rated tow ratings and FMCSA says nothing, and pass right through weigh stations. They cannot, however, exceed any equipment ratings that are stamped in the door jam or ratings on equipment (trailers, hitches, tires, axle ratings, etc). Remember - these mfg tow ratings are NOT stamped on any vehicle plate (door jam) and therefore not enforced by FMCSA or DOT. As a guideline the tow vehicle is designed to stop its GVWR and the trailer is designed to stop its GVWR, and as long you don't exceed equipment ratings you are legal. Again - GCVWR (or tow rating) is not stamped on any door jam and is not enforced (from a legal standpoint).

3 - I have never seen a 3/4 ton truck rated by a manufacturer to tow a 30,000 lbs. Please provide some examples.

 

[Zeronet]

True, the higher numerical gear ratios get the higher tow ratings to limit stress on the transmission and differential. Note how the max-tow differential increases the tow rating. The new 8HP75 transmission in the Hemi is rated for 553 lb/ft of torque. Likely they upgraded the transmission for the 2019 just so it could handle that higher max-tow rating in combination with the max-tow differential?

Note also that the max-tow is only available with eTorque. Probably due to the way eTorque reduces transmission stress during shifts as well as the additional engine braking it can provide.

 

[Zeronet]

3.21 or 3.92 gears will not effect the amount of maximum stress in the transmission.

I disagree to some degree.

It’s harder to accelerate a truck/trailer with the 3.21. The engine will see more load with a 3.21 than with a 3.92.
That load/stress is then transferred through the transmission. So wouldn’t the transmission see more stress?

I know at wide open throttle the torque applied to the transmission input shaft is limited to the maximum engine output. But since the 3.21s are harder to turn,
It seems that the transmission would meet with more resistance to transferring that torque on the output side. Therefore more stress on the transmission.

 

[ExcursionDiesel]

I disagree to some degree.

It’s harder to accelerate a truck/trailer with the 3.21. The engine will see more load with a 3.21 than with a 3.92.
That load/stress is then transferred through the transmission. So wouldn’t the transmission see more stress?

I know at wide open throttle the torque applied to the transmission input shaft is limited to the maximum engine output. But since the 3.21s are harder to turn,
It seems that the transmission would meet with more resistance to transferring that torque on the output side. Therefore more stress on the transmission.

I would think transmission heat would be the result of apply more torque through the 3.21 vs 3.92 to do the same work. Trans temperature is a limiting factor when towing heavy. Anyone with a built diesel knows the transmission is the weak link and will usually fail if not also built. My 4R100 in my Excursion used to routinely overheat until I had it rebuilt by Brian's Truck Shop. It now runs 60-70 °F above ambiant no matter how hard I pull. The ZF in our trucks isn't nearly as stout.

 

[devildodge]

2500 Ram HD
5.7l Hemi...3.73 gear GVWR 9000 GCWR 11180
5.7l Hemi...4.10 gear GVWR 9000 GCWR 13180
6.4l Hemi...3.73 gear GVWR 10000 GCWR 12630
6.4l Hemi...4.10 gear GVWR 10000 GCWR 15630
6.7l Cummins... manual 3.43 GVWR 10000 GCWR 16110
6.7l Cummins...auto 3.43 GVWR 10000
GCWR 17160

Just some numbers for the argument.

You can see the 1500 is pretty competitive with trailer weight...the 2500 can haul though.

And most definitely Engine, transmission and rear gear greatly affect the towing factor of a truck.

The hauling isn't affected much at all and rear gear does not matter.

The 5.7 cannot even be had in a 1 ton dually and is rumored to be leaving the HD Arena.

And the 30000 plus tow ratings are a very specific truck of the 1 ton variety.

 

[NDanecker]

I disagree to some degree.

It’s harder to accelerate a truck/trailer with the 3.21. The engine will see more load with a 3.21 than with a 3.92.
That load/stress is then transferred through the transmission. So wouldn’t the transmission see more stress?

I know at wide open throttle the torque applied to the transmission input shaft is limited to the maximum engine output. But since the 3.21s are harder to turn,
It seems that the transmission would meet with more resistance to transferring that torque on the output side. Therefore more stress on the transmission.

All good discussion points.

You are correct when talking about a specific point in time comparing 2 similar trucks one with 3.21 and other with 3.92. To do the same or equal amount of work to overcome tire friction, wind resistance and/or climbing an incline the stress (or torque) would be less in a 3.92 truck vs. a 3.21 (for example cruising on the highway at 65mph). I was talking about maximum stress, or full throttle acceleration. If you were to do full throttle acceleration on both trucks each truck would then see a theoretical equal maximum stress internally in the transmission (i.e. assuming the output of the motor is the same). Doesn't matter if you have 3.21, 3.92 or how much weight you are towing (resistance). Which one would acceleration quicker? 3.92 of course, and that is why the ratings are higher. The 3.92 can meet SAE specs on acceleration times. In addition a higher numerical gear ratio will help with using the motor to decelerate on declines (inverse of the mechanical advantage the 3.92 gives over the 3.21 for the motor) thus also meeting SAE hill decent requirements (over heating service brakes, run away, etc).

So which is the better gear set for towing? 3.92 or course.

Which will reduce the 'overall' wear-and-tear in the transmission over the life of the truck (or warranty period which is what the mfg is concerned with)? 3.92 of course.

With that said, if you wanted to tow the 3.92 recommended maximum weight with a 3.21 truck you can certainly do it. It won't break the transmission, u-joints, axle shaft, or anything else because each driveline component is designed to withstand the maximum stress generated by the motor. That was the original point I was trying to clarify. Apologize if I misunderstood Chris's original comment.

 

[Chris]

Just some numbers for the argument.

You can see the 1500 is pretty competitive with trailer weight...the 2500 can haul though.

And most definitely Engine, transmission and rear gear greatly affect the towing factor of a truck.

The hauling isn't affected much at all and rear gear does not matter.

The 5.7 cannot even be had in a 1 ton dually and is rumored to be leaving the HD Arena.

And the 30000 plus tow ratings are a very specific truck of the 1 ton variety.

Your right, the reference I found was actually listing a 3500 in it’s 3/4 ton list:
https://www.car.com/buying-guides/best-ton-trucks-10038/

However, I believe the tow rating on that 3500 is transmission limited and won’t increase with a bigger motor, even if a bigger motor will accelerate better.

For the numbers, I think that every motor may have a different transmission/drivetrain/axle setup (I know the diesel does, and the 6.4 has a 1000 Lbs of extra payload which implies a better axle/suspension/something). As such, we can’t use this as direct evidence that a different engine alone significantly effects payload. If it turns out that the extra 1000 lb payload is purely from the engine with no other component upgrades, then I would agree that the engine affects the payload more then I thought and throwing a 6.4 in the 1500 with a 3.21 gear could give you better towing and decent mpg.

Comparing the 1500 to the 2500 is tough due to the transmission as well. We really don’t have any apples to apples comparisons.

The closest I can find is comparing an Ecodiesel to a 5.7; with the 1500 classic quad cab with 3.92 gears, towing increases by about 1245 lbs. this is admittedly a good chunk, but not as much as even the gearing swap from 3.21 to 3.92. The engines also have much different hp/torque ratios and may also have other component changes effecting towing, so it is tough to even use this as a direct example.

I understand that the engine will dramatically effect ride quality as you reduce the struggle on hills and during acceleration (which I think is the ultimate reason the engine options are limited), but my post was geared towards the manufacturer’s rating. I need to dive into SAE J2807 to see how acceleration affects the rating.

I also agree that you can exceed the rating, though you should know what you are doing when you do so and watch the trans temps etc.

 

[devildodge]

and the 6.4 has a 1000 Lbs of extra payload which implies a better axle/suspension/something).

There is no difference in suspension

Comparing the 1500 to the 2500 is tough

I didnt compare 1500 to 2500, I compared Engines, Transmissons, and rear gears of the 2500 Ram HD series.

 

[devildodge]

, I think that every motor may have a different transmission/drivetrain/axle setup (

The HEMI engines are the exact same transmission/drivetrain/axle setup

The Cummins has an optional Manual Transmission and the automatic is different to handle the 800 lbft of torque vs the 425 lbft of torque. Drivetrain and axle setup is the same.

The 6.4l in the 1500 probably wont change those numbers at all...it will just perform better doing it. And most likely it the numbers will go down as the 6.4l will be in a specific truck, just look how much less capable the "street truck editions" are.

Now, 3500...there are alot of different GVWR and GCWR

Check THIS OUT for comparisons

 

[Chris]

There is no difference in suspension

The HEMI engines are the exact same transmission/drivetrain/axle setup

This is confusing; why does the 6.4 with identical components have a signicant GVWR boost, while the 1500 with a v6 (and weaker tranny) has an identical GVWR to it’s 5.7?

 

[devildodge]

This is confusing; why does the 6.4 with identical components have a signicant GVWR boost, while the 1500 with a v6 (and weaker tranny) has an identical GVWR to it’s 5.7?

V6 only has 6900 in 4x4 and V8 has 7100. It is not identical.

2wd is different, but there are a few different reasons.

 

[devildodge]

@Chris also note that the 6.4l is a specific engine for HD.

The 5.7l isn't really setup that way for HD, not an accurate argument, but hopefully you get the jist of what I mean.

 

[SpeedyV]

This is confusing; why does the 6.4 with identical components have a signicant GVWR boost, while the 1500 with a v6 (and weaker tranny) has an identical GVWR to it’s 5.7?

A 6.4L HEMI probably weighs 1,000lb less than the 6.7L Cummins