The 3.43 Rear end

BenK wrote:

Folks...it's always all been about the final gear ratio at pavement vs ICE

So to just reference a diff ratio, today, is out of context...there is
more to it than just the diff ratio...

Absolutely. You have to consider where (in terms of RPMs) the final drive ratios will put the engine at whatever speeds you want to drive.

The final drive ratio is a function of the rear-end combined with the tranny. The best RPM range is a function of the engine. And the speed you want to travel is a function of the driver and the terrain. You have to consider all of these things in order to get an optimal system.

-- Rob

Folks...it's always all been about the final gear ratio at pavement vs ICE

Vs ICE because that then must take into account the ICE's power curve
vs RPM

What is 'new' today are the close ratio trannies that have double OD's and therefore
can have a much lower 1st gear (why I still want a manual 5 speed for my Sub
and it's 4.1's)

Talking just about the diff ratio is now out of context without the new tranny
gear ratios...yes in the 'old' days, was okay enough, as the 1st gears were not
as low (high numeric), nor did they have double OD's

Of course, that then must take into account the other factors...like
tire OD, or better yet, rev's per mile (that is more accurate)

Other stuff like terrain (altitude, inclines, etc), time if year, and a huge ETC

So to just reference a diff ratio, today, is out of context...there is
more to it than just the diff ratio...

bimbert84 wrote:

4x4ord wrote:

The Aisin coupled to a 3.42 provides the following speeds at 1700 - 2900rpm:

3rd ...... 35 - 60 mph
4th ...... 47 - 80 mph
5th ...... 61 - 104 mph

6th gear at 1366 rpm will provide a cruising speed of 60 mph.

So 4x4ord worked out the numbers for this particular combination, and I agree the 3.42 looks ideal. At 70mph, you have 3 useful gears, 4th-6th. Drop your speed back to 60mph or so, and you pick up 3rd gear, which provides 34% more torque than 4th gear.

For comparison, 2900 RPM in 3rd gear with 3.73s would put you at about 55mph. With 4.10s, about 50mph.

-- Rob

I think I am about 2,900 in 3rd at 55mph with 3:42 gears. I pull 6% grades at that speed and rpm.

4x4ord wrote:

The Aisin coupled to a 3.42 provides the following speeds at 1700 - 2900rpm:

3rd ...... 35 - 60 mph
4th ...... 47 - 80 mph
5th ...... 61 - 104 mph

6th gear at 1366 rpm will provide a cruising speed of 60 mph.

So 4x4ord worked out the numbers for this particular combination, and I agree the 3.42 looks ideal. At 70mph, you have 3 useful gears, 4th-6th. Drop your speed back to 60mph or so, and you pick up 3rd gear, which provides 34% more torque than 4th gear.

For comparison, 2900 RPM in 3rd gear with 3.73s would put you at about 55mph. With 4.10s, about 50mph.

-- Rob

bimbert84 wrote:

Wiley75 wrote:

Peak torque of 850 Cummins = 1700 rpm
RPM at 70mph with 3.73 = 1750 rpm
RPM at 70mph with 3.42 = 1590 rpm

There's more to this story. You don't always need peak torque. I'm willing to guess the torque that's available at 1590 RPMs is enough to handle most situations. But when it isn't, you'll have to downshift, and you need to consider where your RPMs will be when you do that. If downshifting one or even two gears pushes the engine out of it powerband, you won't be able to harness that torque multiplication when you need it. I would choose the rear-end that provides useful torque in the greatest number of gears at the speeds you most often drive. Then, on the road, it becomes a matter of selecting the appropriate transmission gear to match the situation.

-- Rob

This is right. 850 lb ft of torque @ 1700 rpm is equivalent to 275 HP. Cruising down the highway while towing your RV will only require a little over 100 HP.

Here is a link to the Aisin gear ratios

While towing you really don't want to be putting much torque through the overdrive gears. So when a hill demands a little more torgue ideal is to start dropping gears. When you are running in 4 th gear the powertrain is most efficient so you want to be able to run many hills in 4th. If need be on the odd very steep hill if you need to drop to 3rd it is not a big deal. The Aisin coupled to a 3.42 provides the following speeds at 1700 - 2900rpm:

3rd ...... 35 - 60 mph
4th ...... 47 - 80 mph
5th ...... 61 - 104 mph

6th gear at 1366 rpm will provide a cruising speed of 60 mph.

Those numbers look perfect to me so long as first gear is adequate to get you going.

When you look at the first gear ratio and multiply things out you can see that 3.75 (first gear ratio) x 3.42(rear end) x 1.9 (multiplication of the torque converter) x 850 lb ft of torque that the engine is capable of yields over 20,000 lb ft of torque on the rear axle .... you come to realize the enormous capability of these trucks and it is easy to see why Ram recommends going with a 3.42 axle ratio for gross combined weights under 29000 lbs.

Wiley75 wrote:

Peak torque of 850 Cummins = 1700 rpm
RPM at 70mph with 3.73 = 1750 rpm
RPM at 70mph with 3.42 = 1590 rpm

There's more to this story. You don't always need peak torque. I'm willing to guess the torque that's available at 1590 RPMs is enough to handle most situations. But when it isn't, you'll have to downshift, and you need to consider where your RPMs will be when you do that. If downshifting one or even two gears pushes the engine out of it powerband, you won't be able to harness that torque multiplication when you need it. I would choose the rear-end that provides useful torque in the greatest number of gears at the speeds you most often drive. Then, on the road, it becomes a matter of selecting the appropriate transmission gear to match the situation.

-- Rob

Gearing is a tough call without having a similar TV to compare for your self. Our 2001 3500 8.1 chev was 4.10. No choice for the dually and it was great. 05 same truck with duramax is 3.73 and again only choice and it works great so now we are talking about lower ratios but more engine hp and torque and heavier fifths than I have so I would really have a problem knowing what would be best. I was thinking the 3.73 would do the job with 850 ft lbs of torque from any engine brand. Using the gears as cummins98 does seems to be ok and then solo driving would be the best for mileage. We all do towing different and different areas of the USA but we all get the job done.
chevman

Don't know what kind of towing you do, but if you look at the GVWR's..all the same now at 14,000# and the only thing that changes is GCWR with whatever gear you go with and it appears that the 3:42 gearing will handle a fairly heavy 5er.....

Don't know why I went with 4:10, more or less based it all on GCWR with the 2010 and the 12' and it carried over to the 13'...guess if I ever decided to get a much heavier 5er, I already have the truck for it, but knowing "after the fact", it appears in my weight range that a 3:42 could have done it well...

Live and Learn? I know that I tow on some very curvy (ESS turn after ESS turn), on these secondary roads that I've towed on in NC, TN, WV and VA, and some quite steep in places. Thus the reason I went with what I already knew would work...If you go to the cummins forum that I post on, there are quite a few that are towing fairly heavy with the 3500 Dually with 3:42 gearing..

Guess you'll have to do the research and figure out what is best for you....I'm looking forward to comparing this truck to my 10 and 12 when it comes to towing the same 5er over the same terrain....will report when I do it...:)

I wish I could pull some grades at the same time another person with the same weight but with 4:10's VS my 3:42.

Funny when I first bought my truck and posted on the TDR my gearing and weights many said the 3:42's were a bad choice and I agreed and planned to swap to 4:10's for $1,800 both axles at Dave Smith. But after towing a lot of mountain passes I felt the combo was doing fine.

Even had the blessing of a high up Chrysler warranty rep. He said "it can't hurt its self" !

If I could go back in time and the 4:10's were available that is what I would have gotten.

So now fast forward to thinking about a 14 Ram with all the bells and whistles I am having a hard time deciding what gearing I would choose. I keep thinking 4:10's but not so sure.

NC can't wait to hear what your impressions are after towing with your new machine.

Cummins12V98 wrote:

Wiley75 wrote:

AlaskanHillBilly wrote:
I have a 07 Dodge and have a 3.73 rear end. I am thinking of trading it for a 2013/14 and have been told that I would get better gas mileage with the 3.43 rear end. I am towing a 14,000 lb 5th wheel. Does anyone know if that is true.

Hey there. I did a bit of research on this topic as well. Here is my opinion:

Given that the torque curve of the new 850 ft-lb Cummins is not flat anymore, and that it peaks out at 1700 rpm rather than 1500 rpm like previous engines, it seems strange to me to have a 3.42 axle ratio. On the other hand, having 4.10 axle ratio, putting the engine way above peak torque, would not be wise either. With the new Cummins/Aisin combination 3.73 would be the best match by far in my opinion. For 2013 you can only get 3.73s in a dually, not sure about 2014s though. (I should note that IF my truck was going to be a daily driver/commuter and would only pull hard a few times a year I might try 3.42s anyway, even if they weren't the best combo for towing)

Peak torque of 850 Cummins = 1700 rpm
RPM at 70mph with 3.73 = 1750 rpm
RPM at 70mph with 3.42 = 1590 rpm

All single rear wheeled 2500 and 3500 2013 Ram trucks have 3:42 ONLY.

All 3500 Ram Dually's are available in 4:10, 3:73 and 3:42.

I am pretty sure this is correct so please correct me if I am wrong.

You're right. One difference also between SRW and DRW 2013 3500's is that the RAWR for the dually is 9750#, for the SRW it's 7,000#, no biggie, but yet a difference... I opted for the 4:10, though believe from all I've read the 3:42 as well as the 3:73 gearing would have sufficed...seeing as how a lot of my towing is on some very curvy mountainous secondary roads, 'tis the reason I went with the 4:10....but as has been stated, believe any would work well unless towing a 5er in the +20,000# range, (22-25,000#)...then, 4:10 I would think would be best.