Dumb question about towing capacities 5.7 Hemi (ex 3.21 v 3.92 gearing)

[lezmark7]

My question is this: when I see that there is a 3,000 lb rating difference between these two rear ends for my Hemi, is that rating due to the fact that the gearing is such that the reduced motor strain with higher gearing makes for a higher tow capacity OR, is it a stronger SYSTEM (parts etc). Simple minds wonder...

 

[AH64ID]

Lower gearing (numerically higher) means the truck has increased leverage to get and keep a load moving.

3.21 and 3.91 are LD ratios and the is a HD forum, but the difference from 3.21 to 3.91 is an increase of almost 22% in rear wheel torque.

 

[lezmark7]

I understand - my question is the gearing the ONLY difference for the higher capacity?

 

[AH64ID]

I understand - my question is the gearing the ONLY difference for the higher capacity?

It is on the HD trucks (aside from DRW vs SRW).

Your sig says 2500, but the ratios you are asking about are not offered on 2500’s.

 

[lezmark7]

Oops, I need to update my profile, traded in my 2500 for a 1500

 

[silver billet]

My question is this: when I see that there is a 3,000 lb rating difference between these two rear ends for my Hemi, is that rating due to the fact that the gearing is such that the reduced motor strain with higher gearing makes for a higher tow capacity OR, is it a stronger SYSTEM (parts etc). Simple minds wonder...

You want my 2 cents?

There is a difference of about 800 pounds in GCWR between the v6 with 3.92 vs v8 with 3.21.

If you've tried to tow with both, you'll immediately discard all tow ratings as "absolutely true and unfailable", because you cannot reduce towing down to a single number. The v6 will feel much weaker vs the v8, certainly much weaker than the 800 pounds suggests.

One of the tests they do in j2807 is 0 to 30 and 0 to 60. The 3.92 will launcher harder from a dead stop, no question, and thats why the 3000 pounds rating is increased. However, if you're on an on-ramp in third gear and need to WOT to get up to 60 mph and merge, both trucks will dig just as hard - or if you're on a freeway trying to pass a semi, again both trucks will be equivalent. The transmission just sits in one lower gear in the 3.21. Remember that you need to multiply transmission gear by axle gear to get your final gear ratio, so you can increase the gear in either spot to get identical torque multiplication; except when both trucks are stopped because then both transmissions are in first so the only play left is the one with the deeper rear axle (3.92).

I think you'll have a hard time finding reports of transmission or axle damage due to towing with either 3.21 or 3.92. I know that's the jist of your question, I wouldn't worry about it. Just treat your truck "normally", take off from a stop no harder than you need to, keep the RPMs above 2000, and you'll be fine towing loads < 8000 pounds. And if you want to tow more than that, you're kinda in the wrong truck anyway as you're going to hit payload and RAWR issues not to mention a soggy suspension.

 

[lezmark7]

You want my 2 cents?

There is a difference of about 800 pounds in GCWR between the v6 with 3.92 vs v8 with 3.21.

If you've tried to tow with both, you'll immediately discard all tow ratings as "absolutely true and unfailable", because you cannot reduce towing down to a single number. The v6 will feel much weaker vs the v8, certainly much weaker than the 800 pounds suggests.

One of the tests they do in j2807 is 0 to 30 and 0 to 60. The 3.92 will launcher harder from a dead stop, no question, and thats why the 3000 pounds rating is increased. However, if you're on an on-ramp in third gear and need to WOT to get up to 60 mph and merge, both trucks will dig just as hard - or if you're on a freeway trying to pass a semi, again both trucks will be equivalent. The transmission just sits in one lower gear in the 3.21. Remember that you need to multiply transmission gear by axle gear to get your final gear ratio, so you can increase the gear in either spot to get identical torque multiplication; except when both trucks are stopped because then both transmissions are in first so the only play left is the one with the deeper rear axle (3.92).

I think you'll have a hard time finding reports of transmission or axle damage due to towing with either 3.21 or 3.92. I know that's the jist of your question, I wouldn't worry about it. Just treat your truck "normally", take off from a stop no harder than you need to, keep the RPMs above 2000, and you'll be fine towing loads < 8000 pounds. And if you want to tow more than that, you're kinda in the wrong truck anyway as you're going to hit payload and RAWR issues not to mention a soggy suspension.

I got rid of my 2500 HD diesel because I bought it to tow my 8400 lb trailer and we havent towed in a couple years. That truck ride as a daily vehicle just wore me out. Wife is pissed but she doesnt have to drive around town every day in it. If we dont sell our trailer, I am just wondering how big a deal it would be to take periodic road trips. If we did, it would be flat land (not like driving thru the Sierras like we used to). In Texas now and would likely head east and not up and down through mountains. My thought is that keeping tanks empty and just being generally aware of weight that IF we towed again and didnt push, I would likely be OK. Hence my question. If the only real difference is gearing, seems to me with 8 speeds i could minimize engine work.

 

[lezmark7]

I got rid of my 2500 HD diesel because I bought it to tow my 8400 lb trailer and we havent towed in a couple years. That truck ride as a daily vehicle just wore me out. Wife is pissed but she doesnt have to drive around town every day in it. If we dont sell our trailer, I am just wondering how big a deal it would be to take periodic road trips. If we did, it would be flat land (not like driving thru the Sierras like we used to). In Texas now and would likely head east and not up and down through mountains. My thought is that keeping tanks empty and just being generally aware of weight that IF we towed again and didnt push, I would likely be OK. Hence my question. If the only real difference is gearing, seems to me with 8 speeds i could minimize engine work.

BTW - I just love the ride and this new truck in general!!

 

[silver billet]

I got rid of my 2500 HD diesel because I bought it to tow my 8400 lb trailer and we havent towed in a couple years. That truck ride as a daily vehicle just wore me out. Wife is pissed but she doesnt have to drive around town every day in it. If we dont sell our trailer, I am just wondering how big a deal it would be to take periodic road trips. If we did, it would be flat land (not like driving thru the Sierras like we used to). In Texas now and would likely head east and not up and down through mountains. My thought is that keeping tanks empty and just being generally aware of weight that IF we towed again and didnt push, I would likely be OK. Hence my question. If the only real difference is gearing, seems to me with 8 speeds i could minimize engine work.

Very hard to say, if that's 8400 pounds dry then you're going to exceed GCWR with the 3.21. But yes it definitely helps if you plan to tow flat and take it slow and easy as the ratings are done under some pretty tough conditions which you may never see (towing up the davis dam).

SAE J2807 Tow Tests

Here is some helpful info explaining how SAE J2807 tow tests are performed

fifthwheelst.com

 

[Turin]

In the 1500s, the difference between the 3.21 and 3.92 axle ratios is basically trading a high gear for a low gear. The final drive ratios for 3.92 gears 2-8 are basically the same as 3.21 gears 1-7. With the 3.92 you get an extra low gear (good for getting a heavy load moving). With the 3.21 you get an extra high gear (good for highway gas milage). The gearing is the only thing behind the different tow ratings.

I got rid of my 2500 HD diesel because I bought it to tow my 8400 lb trailer and we havent towed in a couple years. [...] If we dont sell our trailer, I am just wondering how big a deal it would be to take periodic road trips.

Is that the GVWR of the trailer or the dry weight? Assuming GVWR, it's almost exactly what I pulled with my 1500 which had the 5.7 Hemi (non eTorque) and 3.92 rear axle ratio. I also switched from P-metric tires to LT tires. That hurt the ride a little but improved towing. For local trips (shorter and slower) you should be fine, though you'll have to watch payload. Longer trips are possible but depending on conditions (road/wind/traffic) they may not be comfortable. You'd be pushing the limits of your truck, so make sure you have a weight distributing hitch with good sway control.

 

[Firebird]

I got rid of my 2500 HD diesel because I bought it to tow my 8400 lb trailer and we havent towed in a couple years. That truck ride as a daily vehicle just wore me out. Wife is pissed but she doesnt have to drive around town every day in it. If we dont sell our trailer, I am just wondering how big a deal it would be to take periodic road trips. If we did, it would be flat land (not like driving thru the Sierras like we used to). In Texas now and would likely head east and not up and down through mountains. My thought is that keeping tanks empty and just being generally aware of weight that IF we towed again and didnt push, I would likely be OK. Hence my question. If the only real difference is gearing, seems to me with 8 speeds i could minimize engine work.

If your truck is a 3.21 geared 1500, then the quick answer is no

 

[Redfour5]

I understand - my question is the gearing the ONLY difference for the higher capacity?

No difference... But the difference there is, is noticable but not dramatically when hauling under 8 to 9K with the ZF transmission. I had both a 2015 3.92 5.7 and a 2021 3.21 5.7. They both towed a 7K travel trailer pretty well if you don't mind the trailer occasionally very occasionally deciding to run the show for a few seconds... My 2500 is planted in a way a 1500 could never be and the 6.4 is a major difference. But in terms of towing below max by about 20 to 25%, with the 3.21, the ZF tranny compensates for the lower gear ratio by working harder than with the 3.92 where the gearing does more of the work. It is noticable but not in a bad way from the towing standpoint. They both tow about the same just differently... Of interest, my 2013 quad cab still had the six speed tranny with a 3.55 and 5.7 and that was a very good combo for towing and get up and go like passing on a two lane road... I'm actually surprised at them pretty much getting rid of the mid range gear (except for the Pentastar 1500.) I'm also surprised that the 4.10 gears aren't standard on the 2500's with the 6.4 and ANY towing packages... They are able to put in a number of defaults on the build site, why not that one. You want the tow package or the tow set up in the bed or the air suspension they should default to the 4.10 imho. I'm also surprised that the 4.10 gears aren't standard on the 2500's with the 6.4 and ANY towing packages... If you want one off the lot, it's the rare dealer who understands enough to order a 4.10 rear end that costs surprisingly little if ordered vs the 3.73. That ought to be the norm in my opinion, but the 3.73 does an excellent job under 10K towing imho AND, if you want "beefed" up other aspects for a truck, that's why you go HD (2500/3500).