Are you sure somebody didn't sneak a Powerjoke underneath the Hood ???
The problems you are having with your 2001 truck is, Low power, Low Mileage.
Low Power, Low Mileage. . I get those symptoms real bad with Boost leaks and clogged air filter. Can you say 9 MPG empty?
If EVERYTHING is buttoned up real tight, and I have a new/good airfilter, I get ~15MPG empty, in the city. I have a "heavy 2wd 3500,, camper, grill guard, Junk in the Bed,, Junk in the cab,, including a rather "nice" stereo

atleast to me...
I get alot more mileage on the highway,, all the way up to 25MPG@~60MPH, I can get 20MPG on the highway when I want to,, the 25MPG was kinda a "freak incident" I must have had a tailwind. .
You should be able to get 15-18MPG empty, and 12 towing. These are "normal" numbers for a 24V
If you want Higher MPG,, DON'T LUG!!, This is the First thing Bill K. and most others Will tell you when asking about MPG,, don't tow in Overdrive,, even though the 12V motors loved to work WOT@1,500RPM the 24V is a very different beast. .
If you don't have Gauges,, WHY NOT?? Get Some!
If you are worried/talking about milage, at least get a boost gauge,, drive with the least amount of boost.
I have nearly the same truck as you do. just an earlier model.
I have a hill close to my house,, it's pretty steep, maybe a 6% grade (steep for flatland) if I am doing 50MPH in 5th gear, I run about 15+ pounds of boost empty, if i am at the same speed in 4th gear, I can actually accelerate up the hill, and not get 15 Pounds of boost. Less Boost equals More MPG. The optimum RPM for the 24V is 2K-2. 5K Less is lugging and more,, is better than lugging.
What I'm saying is,, Use 4th gear, even though the engine is screaming at 2,600-2,700RPM You will get more power, and better MPG than using indirect overdrive (5th) gear.
The dealer won't do much about MPG with yuor truck,, know why?? Because there is no MPG established for your truck If you are getting anything over 0 MPG they are (dealer) aren't worried about it. A Cummins shop would definatly be worried, but what I am saying is there are no
NUMBERS to go by. Whatever you get is what they are supposed to get kinda thing. Look at your windowsticker, and you'll see what I mean.
Low Power,, Power is a mix, and is very relative. Power also comes in different forms.
When comparing the 12V to a 24V,, the 24V seems to be a Sleeping bear. In all reality,, it is a sleeping bear,, below 2K RPM the motor is nearly dead, and gets only "Marginal" MPG under load. (un-loaded is a different story)
When towing, try to your best to keep RPM over 1,500RPM at all times 2K-2,500 RPM will give more power and more MPG than 1,500RPM.
Ever notice how neck snapping the acceleration is in 1st gear at WOT ? As the gears progress,, 1,2,3,4,5 the gear with the least amount of acceleration for amount of power is going to be,, you guessed it,, 5th gear. It's gonna take ALOT more fuel to maintain speed in 5th, then 4th. The power you don't have to use in 4th gear is what offsets the MPG used with more RPM, compared to 5th gear.
Anything below 1,500RPM is also Torture on the drivetrain, and clutch!! I never even thought twice about going to WOT at 1,500RPM,, Now I have DD1 injectors, and a PE COMP,, I can easily slip my stock clutch at 1,500 RPM and get lots of driveline noise,, where,,making even MORE power at 2,000-2,500RPM my clutch won't slip and I the power is smooth.
My Dad has owned three Dodges,, and ALL,, including mine,, towed excessivly, and excessivly over all GWR's (GVWR, GCWR, etc. ) the '91 Cummins he had got excellent MPG,, and loved the Low RPM. We had to replace the clutch at 75K, we gave it to a needing family member at that time also.
My Dad's 2nd Ram was a '96 12V. That motor liked the low RPM also,, it could tow all day, and get 14-17mpg but wouldn't get above 70MPH We put 120K on that truck before the clutch started slipping a little when taking off.
My '98. 5 so far has been our best Dodge,, It got 12-14MPG towing,, and can tow at any speed we want too. 85MPH if we dared. I still have the original clutch, and the 5th gear nut. My Dad never towed in 5th gear,, NEVER! That Cummins would be spinnnin to what sounded like 5K RPM,, but is was really only 2. 7-2. 8K That's when he would get 14MPG, in 5th gear, he would go down to 10MPG (that's why he never towed in 5th gear)
The trailer he towed was a 25Foot 25,000GVWR Gooseneck Tandem Axle, Dually. The trailer alone grossed in between 19-30K.
He now has a Duramax with Allison transmission. I get 7. 2 MPG when I tow, and he gets about 8MPG. We tow a "lighter" trailer now. a 20' Tandem Axle, Dually that ways about 16-25K
Hope I helped,, The post was getting long,, I'lll stop now.
MerrickNJr
P. S. A little money spent on your truck can go along way, don't be afraid to customize it,, this is what makes YOUR truck YOURS!!
Just look at mine.

Now that's MY truck. Not Cookie Cutter.
