Good Sam Club Open Roads Forum: Why diesels are most efficient around 1,800 rpm
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 > Why diesels are most efficient around 1,800 rpm

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Cummins12V98

on the road

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Posted: 10/23/20 11:43am Link  |  Quote  |  Print  |  Notify Moderator

This is my fourth season double towing. I drive by the troopers all the time or they slowly pass right along side of me. Both WA and OR Double towing is not allowed. BUT I am licensed SD so who knows that may help. CA recognizes SD so that is why I am fine towing doubles there also.

It costs $500 each way to have my MC transported. So I am to the good over 3k at this point. If pulled over I will simply separate and leave the MC and trailer with someone and continue to the next State and drop RV and come back to pick up the MC.


2015 RAM LongHorn 3500 Dually CrewCab 4X4 CUMMINS/AISIN RearAir 385HP/865TQ 4:10's
37,800# GCVWR "Towing Beast"

"HeavyWeight" B&W RVK3600

2016 MobileSuites 39TKSB3 highly "Elited" In the stable

2007.5 Mobile Suites 36 SB3 29,000# Combined SOLD

4x4ord

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Posted: 10/23/20 04:53pm Link  |  Quote  |  Print  |  Notify Moderator

Sounds like this guy likes to tow in 5th for many of the same reasons as Shiner but he does indicate he gets slightly better fuel economy in 6th. He might have slightly bigger tires? Seems like splitting hairs to me. I’m certainly going to go for the quieter cab with the lower rpm if I can save a few pennies at the pump. If the transmission needs to be babied to handle towing light why not just buy a Ford?

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ShinerBock

SATX

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Posted: 10/23/20 08:48pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

Sounds like this guy likes to tow in 5th for many of the same reasons as Shiner but he does indicate he gets slightly better fuel economy in 6th. He might have slightly bigger tires? Seems like splitting hairs to me. I’m certainly going to go for the quieter cab with the lower rpm if I can save a few pennies at the pump. If the transmission needs to be babied to handle towing light why not just buy a Ford?

Link


Slight? They look like the same size tires as my truck which are 34s. That would put him at 2 inches taller than stock for that truck. But of course it seems like splitting hairs to a Ford fanboy who thinks he knows better than Cat, Cummins, and every other diesel manufacturer engineer just so they can piss on other truck makes.

Also, guess what rpm a 4.10 Ram 3500 that is in 6th to get that good mileage when towing? The same rpm I am at in 5th with 3.42s. Who da thunk it.....

As for your link, the guy is only towing 15k combined which is within the 5-7k range were I stated that 6th is more economical for me earlier in this thread. I am towing another 6-7k lb toy hauler 5ver with a taller than normal roof. It is not necessarily due to a weak trans, but rather that a tall rear gear combined with a tall OD gear puts more stress on trans clutches than a shorter rear gear ratio would. This goes for any transmission with a tall rear gear combo towing heavy.

* This post was last edited 10/23/20 09:42pm by ShinerBock *   View edit history

4x4ord

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Posted: 10/23/20 11:47pm Link  |  Quote  |  Print  |  Notify Moderator

Did you notice that by dropping from 6th to 5th and slowing down from 65 mph to 55 mph he said his engine rpm increased from 1401 to something like 1565. Cummins says slowing down should have gained him 1 mpg for the lower wind drag and Shiner says he should gain some fuel economy speeding his engine up a bit. Combined you’d expect a fairly significant gain. Turns out it was only a gain of .4 mpg. I thought the Cummins rock solid rule of gaining 1mpg sounded reasonable but my take away would be that the gear drop hurt a bit so the net gain was only .4 mpg.

Vintage465

Prunedale CA.

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Posted: 10/24/20 07:58am Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

Shiner, you are applying a little bit of truth improperly to come up with a stupid recommendation.

I couldn't read your post without at least doing a simple test to give some real somewhat scientific numbers so:

I drove my truck in 5th gear at 1800 rpm/92 kph up a long incline and measured the fuel economy on the DIC .... 13mpg (18.6 liters per 100). I turned around, went back and conducted the same test at 92 km per hour in 6th gear (1414 rpm) and measured 13.6 mpg (17.8 liters per 100). I then did a similar experiment over a stretch of highway going down a grade for the duration of the test... 43 mpg in 6th at 92 kph (1414 rpm) and 33 mpg in 5th at 1800 rpm/92kph. So although a diesel engine may be most efficient at 1800 rpm it is best to let the engineers worry about what gear and rpm your truck should run at. Put it in "D" and go. And choose the highest speed (lowest numerical) rear end recommended for the load you expect to tow.


About the time you used "stupid" in your reply I lost all interest in your comment. In the big scheme of things all these forums are for fun and info. When people start using offensive words like "stupid" the thread has no place to go but down. I have yet to find one person that really enjoys being told what they believe or are discussing is stupid. Not sure why we all can't be more careful about the words we choose to use.
And........I found Shiners info enlightening. Everything is happiest at 1800. Not everthing gets done at 1800, but mine is happiest.


V-465
2013 GMC 2500HD Duramax Denali. 2015 CreekSide 20fq w/450 watts solar and 465 amp/hour of batteries. Retiring in 2021, then look-out road, here we come!

4x4ord

Alberta

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Posted: 10/24/20 08:35am Link  |  Quote  |  Print  |  Notify Moderator

Yup that was kind of stupid of me ..... sorry Shiner if you took offence. I’m certain if Shiner took a ride in one of our big trucks he’d see those engines are happier at 1300 rpm than they are at 1800 under light loads. Certainly the operator is happier.

Cummins12V98

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Posted: 10/24/20 09:39am Link  |  Quote  |  Print  |  Notify Moderator

"Also, guess what rpm a 4.10 Ram 3500 that is in 6th to get that good mileage when towing? The same rpm I am at in 5th with 3.42s. Who da thunk it....."

I loved my 3.42's in my 11 HO RAM DRW. Only issue was I had to take it easy on launch on grade. Fast forward my 15 has 4.10's and AISIN. Hind sight I should have bought the 3.42's with AISIN and towed in 5th same rpms I now do in 6th. The LOW starting gears in the AISIN would have made launching 36k a non issue and had another gear for better highway economy running solo.

SweetLou

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Posted: 10/24/20 10:57am Link  |  Quote  |  Print  |  Notify Moderator

Cummins12V98 wrote:

"When towing, whatever gear keeps engine rpm between 1,800 and 2,000 rpm will give the best fuel mileage and power because that is the rpm window where the Cummins 6.7-liter makes peak torque.

I tow at 60 with rpm's about 1,750. Just right! That's with 4.10's. At 70 I am about 2,000.

With my last DRW I had 3.42's. I would tow in 5th at same rpm's as above, just right!

Exactly right


2013 3500 Cummins 6.7 Quadcab 4x4 3.73 68FE Trans, 2007 HitchHiker Discover America 329 RSB
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4x4ord

Alberta

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Posted: 10/24/20 11:41am Link  |  Quote  |  Print  |  Notify Moderator

SweetLou wrote:

Cummins12V98 wrote:

"When towing, whatever gear keeps engine rpm between 1,800 and 2,000 rpm will give the best fuel mileage and power because that is the rpm window where the Cummins 6.7-liter makes peak torque.

I tow at 60 with rpm's about 1,750. Just right! That's with 4.10's. At 70 I am about 2,000.

With my last DRW I had 3.42's. I would tow in 5th at same rpm's as above, just right!

Exactly right

I’ll expect to be getting the 3:31s in my next truck. Whether I’ll have to lock out 9th or tenth will depend on the load I’m pulling but with 1050 lb ft of torque most my loads will be considered light.

FishOnOne

The Great State of Texas

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Posted: 10/24/20 03:16pm Link  |  Quote  |  Print  |  Notify Moderator

SweetLou wrote:

Cummins12V98 wrote:

"When towing, whatever gear keeps engine rpm between 1,800 and 2,000 rpm will give the best fuel mileage and power because that is the rpm window where the Cummins 6.7-liter makes peak torque.

I tow at 60 with rpm's about 1,750. Just right! That's with 4.10's. At 70 I am about 2,000.

With my last DRW I had 3.42's. I would tow in 5th at same rpm's as above, just right!

Exactly right


Exactly wrong.

The 6.7 cummins only makes it's peak torque at wide open throttle and full boost at that rpm and I'm pretty sure no one drives down the highway at wide open throttel.


'12 Ford Super Duty FX4 ELD CC 6.7 PSD 400HP 800ft/lbs
"200k Mile Club"
'16 Sprinter 319MKS "Wide Body"


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