103 posts in this topic

On 1/8/2016 at 9:38 AM, Moses Ludel said:

RuebenT...Considering all of the features and mods on the truck and engine, the injectors must be the issue.  The BD programmer or even the stock ECM would want to pulse the injectors at the stock injector flow rate.  The high output injectors would be pouring more fuel per each pulse of the injectors.  The poor fuel mileage and black smoke reflect this during your run-up through the gears.

I would also suggest that 2000 rpm is a bit high for maximum fuel efficiency, though 14 mpg would not be the result of your 2000 rpm shift points or cruise rpm.  For maximum fuel efficiency with the 6-speed's gear spacing, I would upshift at 1250-1400 rpm with no load and 1600-1700 rpm with the trailer in tow.  Note the mileage increase.  

This engine has a prompt torque rise and makes peak torque (stock) by 1600 rpm.  Especially with the six-speed transmission, running between 1600 and 1900 rpm at flat ground cruise speeds in overdrive, your truck should achieve at least 19-21 mpg running empty.  Your trailer load is not excessive for the 5.9L Cummins engine, towing should get 14-17 mpg.

Gearing would be another concern if you were running greatly oversized tires.  You're not.  If you have stock axle gearing of 3.73 or 4.10 ratio with the 285/70x17 tires, 20-plus mpg should be attainable without a load.

One thing for sure, the factory mpg calculator is way off, and that's typical.   Like you, I find that the only way to clearly know mileage is to run an accurate and specific number of miles (known road distance); begin with a full tank of fuel and refill at the end.  Calculate your mileage from there.

Moses

 

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Hi Ludel I have a 2006 Dodge Ram 2500 4X4 5.9L. My truck has 3:73 gears and 35 Toyo M/T 2 " level kit. I drove to Macon Ga. from Tampa Fl.@ 65mph and got 28 Mpg up I75 in March of this year. I am lifting my truck 6" and putting 37" Toyo MT on. I was going to put 4:30 Nitro Gears in. I also got 25-26 MPG from here to NC in Dec. of 2014. My MPG around town is 16-18 city driving. Now all of this is empty truck bed. Now i do plan on towing a camper to Ga every now and then and any where else i go for vacation. I was wondering being that my RPM's are around 1600 to 1700 at the speed mentioned above would i receive equal results  doing my lift and gear change. I used a RPM calculator and for the given size tire the rpm's for a 4:10 set up were 1829 rpm and for a 4:30 set up 1913 rpm. Now i compared that to the set up I have now using the Calc. and my rpms are 100 rpm higher with the 4:30 set up and lower with the 4:10 set up than i currently have. I think you are reporting that the 5.9L sweet spot for fuel maximizing is around 2000 rpm, because you want the higher rpm range when traveling at 70 mph + to avoid high boost pressures. 

  Oh my calculations were done at 70 mph.  If I go to a 4:56 set up the rpm's go over 2000. Can you help. with a bit of knowledge.

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4x4 Firefighter...Welcome to the forums!  The sweet spot for maximum fuel efficiency is the 1600-1700 rpm range for a 2006 stock tune 5.9L Cummins engine.  The 2000 rpm figure is my maximum torque point with an aftermarket software program.  Your stock tune torque peak is at 1,600 rpm.  As a rule, the peak torque point is the maximum fuel efficiency sweet spot.  Even with my software tune, however, best fuel efficiency is still in the 1600-1900 rpm range.

Our original axle gearing was 3.73.  As you can see, I did the 4.56 gears, which suit our trailer towing.  We have an 8400# (loaded) travel trailer and plan to use it regularly.  When trailering (or at any other time), if I hold the speed to 62-65 mph, I can squeeze reasonably good fuel efficiency from this hefty package.  Driving past 65 mph, there is a linear, rapid decline in fuel efficiency.  Were I to do the gearing all over again for my 34.6" diameter tires, I would likely go with 4.30 rather than 4.56 gears.  (4.30 was not available at the time I performed the change-out.)  If we rarely towed, 4.10 might be a consideration, as this would be an exact correction from the stock diameter tires with 3.73 gearing to 34.6" tires.

Cummins recommendations for commercial use (up to 50K GVW) of the ISB sixes is 2,100 to 2,400 rpm for "maximum economy".  This accounts for the typical commercial/medium duty truck with a van box or other "billboard" size wind resistance factors plus much heavier loads than a Ram light truck (to 1-ton) might experience.  I can assure everyone that operating our lighter trucks at a sustained 2100-2400 rpm will burn considerable fuel.  In my case, with the 4.56 gears and 34.6" tires, the loss of fuel efficiency is not a reflection of extreme loads but rather the fuel needed to propel the truck at 70-plus mph.  A friend and former Chrysler VP of engineering has suggested that I stick to my original plan, keep the 4.56 gears, and hold the truck to 65 mph if I'm interested in maximum fuel efficiency.  His emphasis is that pushing this kind of poor aerodynamic mass down the road at high speeds is a recipe for burning fuel.

In my view, plans for an even taller truck (i.e., approaching billboard wind resistance with a camper), running 37" tires, would call for at least the 4.30 gears for camper use at the speeds you describe.  There are weak links in these powertrains, in particular the 48RE automatic. I'm on the original 48RE factory transmission at 150K miles*, and it's still working very well.  This in part is because I've relieved the transmission loads with the 4.56 gearing.  Stock 3.73 gears with 37" tires would take out the transmission quickly.  4.10s with 37" tires will not restore the original gearing.  4.30s with 37" tires would come very close, offering the closest equivalent to stock tire diameter (31.9") and the stock 3.73 gears.  4.56 would even be acceptable, though if you only plan to tote the camper occasionally, you would have better fuel efficiency running empty with the 4.30s.

*Note: I made valve body changes as per Sonnax recommendations, you can see at the magazine site:  http://www.4wdmechanix.com/Survival-Upgrades-for-Jeep-and-Dodge-Ram-Automatic-Transmissions?r=1.  This has helped extend the original 48RE transmission's life.

In my experience, the larger 37" tire diameter and a raised truck will impact wind resistance dramatically.  You will likely never see 28 mpg again, even with 4.30 gearing.  24-25 mpg will also be daunting.  My best totally stock fuel efficiency with our truck was 25 mpg at a sustained 1600-1750 rpm over a 500 mile test.  I subsequently chassis-lifted the truck 4", added 1100# of combined accessories, auxiliary fuel and oversized 35" tires and rims with 4.56 gears, installed a bold (i.e., no longer aerodynamic) front winch bumper and 18K Superwinch, and I have never achieved 25 mpg since!  At 65 mph on level ground, I can generally expect 21-22 mpg running empty without the trailer these days.

Moses

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