Revert to a stock tune. You'll get better mileage......

 

Yes, easy mod is to put a set of vertical seals off a ~2014 & up 1500 on the bed... there is a place for the seal to slide on and the install takes seconds per side... Cut the seal so it stops at the body line on the side of the truck, the one that leads to the top of the rear bumper. Anything lower will cause drag as the airflow will go into the bumper. The seal is tucked in between the cab and the bed, out of sight unless you go look for it.



and if your really motivated...

MPG mod info (consolidating info into one spot) | Dodge Cummins Diesel Forum (cumminsforum.com)

FWIW, my truck is getting 21~22 mpg with peaks into the 23's on 500ish mile tanks all year.

 

Greeting,
I owed a 1986 Dodge D-100, 318, 3 Spd Auto..Owned it for 13 years, nothing you could do increased the performance or gas milage, change the timing by 1/2 degree the truck would not run right. 12.5 city & 16 Highway.
My wife owned a 2004 Ram 1500, Work Package truck, V-6, 4 Spd Auto., for 18 years.9 to 10 City & 13 to 14 Highway.
This brings up to my 1999, Ram 2500, V-10, 4 Spd auto, 2WD, 4 door Quad. I purchased recently at an Illinois farm auction. 91,000 miles, perfect interior, absolutely no rust, but a few dings and scratches. I replaced the original Spark plugs, wires, Throttle Position Sensor, Idle Air Control Valve and Mass Air Sensor, the V-10 runs great. 9 mph city & 14 highway.

 

488 cubic inches are THIRSTY. If I can get my average to break double digits, I feel I am doing good. Mine has 4.56 gears, and 33" tires though.

 

I get 16.4 mpg and I have a 2007 Dodge Ram 1500 Bighorn edition with a 2009 5.7 liter hemi motor

 

1996 1500 Ram Pace truck 5.9, auto, between 13 and 17mpg, I put 17" wheels on it (stock are 16") and seemed to help a bit.

 

Dropped from 12-13 mpg for most it's life to 10mpg in this last year. 134,00 miles on her now and I am trying everything to get back two or three I lost with no luck yet. Any advice is welcome.

 

How old are the O2 sensors?

 

with only 75k, I bet they are original ones. you can get a bluetooth obd2 reader and use the torque app to figure out what is going on with the cpu. look at fuel trims and vacuum.... if the fuel trims are off, there is a reason and you can discover it with that app.

 

They are most likely OE. I can't find any record of them getting replaced. I have replaced the plugs, coils, and wires. Also the PCV and Purge valves. I will try those O2 sensors next.