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 > 6.4 litre HEMI vs Cummins

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samhain7

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Posted: 09/29/20 10:09pm Link  |  Quote  |  Print  |  Notify Moderator

Hi all
Just upgrade TV from a Dodge 2500 6.4 litre to a 2500 Cummins
Looking to see if anyone else made this change to share your experiences on gas mileage.
Did the cummins make a difference in regular driving?


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ShinerBock

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Posted: 09/30/20 05:45am Link  |  Quote  |  Print  |  Notify Moderator

The Cummins gets horrible gas mileage.... It does get good diesel mileage though... [emoticon]

Expect about 15-16 mpg(calculated) combined depending on how you drive and terrain. The computer may say 17-18, but it lies.

Also, when you tow over 5k, lock out 6th at speeds under 73 mph for best fuel mileage if you have a 3.42 rear end.

* This post was edited 09/30/20 05:56am by ShinerBock *

spoon059

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Posted: 09/30/20 06:25am Link  |  Quote  |  Print  |  Notify Moderator

The term you want is FUEL mileage. Yes, the Cummins will get better mileage than the 6.4 in almost all conditions. I get a little better than Shiner, I get about 17-18 around town and can get into the low 20s on the highway. Towing a 10K lbs travel trailer at 68 mph I get between 11-12.5 usually. Lots of mountains and it will get a little lower, a nice tailwind and it will get a little higher.


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rhagfo

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Posted: 09/30/20 06:30am Link  |  Quote  |  Print  |  Notify Moderator

You will notice a slight difference in towing power. [emoticon]


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mudfuel07

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Posted: 09/30/20 07:25am Link  |  Quote  |  Print  |  Notify Moderator

Like Spoon, I get around 17-18. I came from a 5.7 Tundra and love the power. I get around 8-9 towing our 10k trailer, but I also have a little larger tires. Mine feels like it's a gas engine with the tune I have, so no difference in daily driving.


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IdaD

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Posted: 09/30/20 07:40am Link  |  Quote  |  Print  |  Notify Moderator

I average 15-16 in my typical city/suburban driving. Highway unloaded over 20 is easy but that's a pretty small share of my miles. Towing roughly 20k combined is 9-12 depending on conditions and my level of patience. I'm at almost 70k miles and I've had a failed water pump and one bad nox sensor I had to replace.


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Grit dog

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Posted: 09/30/20 09:45am Link  |  Quote  |  Print  |  Notify Moderator

samhain7 wrote:

Hi all
Just upgrade TV from a Dodge 2500 6.4 litre to a 2500 Cummins
Looking to see if anyone else made this change to share your experiences on gas mileage.
Did the cummins make a difference in regular driving?


Congrats on the upgrade. Unless you bought it today would this question not automatically answer itself for you in very short order? Possibly immediately by using that fuel mileage thingy that comes on virtually all newer vehicles or some simple math after running through a tank of fuel?
(If you’re asking about mileage after breakin on a new vehicle, I understand, but you didn’t ask that and there’s probably 1,234 Cummins fuel mileage threads on this forum alone and 12,345 threads about it if you include other popular internet opinion sites.)

He!! There’s one on the front page right now live and in person with the opinions of info you requested.


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Me Again

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Posted: 09/30/20 06:17pm Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:

The Cummins gets horrible gas mileage.... It does get good diesel mileage though... [emoticon]

Expect about 15-16 mpg(calculated) combined depending on how you drive and terrain. The computer may say 17-18, but it lies.

Also, when you tow over 5k, lock out 6th at speeds under 73 mph for best fuel mileage if you have a 3.42 rear end.


I have been towing the 8k TT in 6th without TH and getting better mileage. In 5th the 16K 5th and 8K TT got similar mileage.


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ShinerBock

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Posted: 09/30/20 06:58pm Link  |  Quote  |  Print  |  Notify Moderator

Me Again wrote:

ShinerBock wrote:

The Cummins gets horrible gas mileage.... It does get good diesel mileage though... [emoticon]

Expect about 15-16 mpg(calculated) combined depending on how you drive and terrain. The computer may say 17-18, but it lies.

Also, when you tow over 5k, lock out 6th at speeds under 73 mph for best fuel mileage if you have a 3.42 rear end.


I have been towing the 8k TT in 6th without TH and getting better mileage. In 5th the 16K 5th and 8K TT got similar mileage.


It is speed and load depended. As you can see the BSFC map below. The "sweet spot" for the Cummins 6.7L under load is between 1,600 and 1,900 rpm. The red dots/lines is 6th gear and the four vertical lines staring from the left is my rpms at 60, 65, 70, and 75 mph while what I tow. The yellow dots/lines is my rpms in 5th at the same speeds. The horizontal lines are grade/load starting with 0% grade at the bottom.

On completely flat ground and no drag, 6th is generally best, but even a slight 1% grade and moderate drag will put enough load on the engine to make 5th the better choice which is why I generally leave it in 5th and forget it unless I am going past 72-74 mph.

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spoon059

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Posted: 10/01/20 08:08am Link  |  Quote  |  Print  |  Notify Moderator

ShinerBock wrote:


It is speed and load depended. As you can see the BSFC map below. The "sweet spot" for the Cummins 6.7L under load is between 1,600 and 1,900 rpm. The red dots/lines is 6th gear and the four vertical lines staring from the left is my rpms at 60, 65, 70, and 75 mph while what I tow. The yellow dots/lines is my rpms in 5th at the same speeds. The horizontal lines are grade/load starting with 0% grade at the bottom.

On completely flat ground and no drag, 6th is generally best, but even a slight 1% grade and moderate drag will put enough load on the engine to make 5th the better choice which is why I generally leave it in 5th and forget it unless I am going past 72-74 mph.

Thanks for the graph Shiner. I've wondered which is more efficient, but I haven't been able to test it personally. Our regular trip is to a lake in PA, so we deal with traffic and mountains. Too many variables to get a competent comparison. Our yearly Florida trip is about 2000 miles round trip, but I always forget to document my towing mileage separate from my "around town" mileage to compare it.

It makes more sense to have slightly higher RPMs and maybe a little less turbo pressure to get better fuel economy.

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