In most cases the farther you deviate from how a newer vehicle came from the factory the less mileage you'll experience. These things are set up for optimum mileage and performance with the thought of obtrusive noise being kept out of the equation. Many people think that open exhaust will improve performance and mileage which just isn't the case more often than not. The computer reads everything happening with the engine and when something is out of acceptable range compensates to try making up the difference. Too much exhaust flow will normally appear as a lean condition and so the computer then sends more fuel to richen the mix. Put some sort of exhaust back on it with the cats and new 02 sensors. Mine is 2wd but normally returns 16 city and 18-20 highway (I drive fast).

 

Thanks for the info.. Did your proformance Increse without the CAT?? (Acceleration/Toruque)

 

Always amazes me how many folks believe they can improve on the manufacturers $billions invested in R&D by changing anything the factory put in any vehicle and expect there will not be a negative result. Want more performance in the 6 ? Make changes to any part of the engine/exhaust and resolve yourself to a reduction in mileage. OR,, peddle the 6 and go shop'n for a Dakota with a whump'n V8 - a HEMI would be Awsum!! There's tons of v8 Dakota's out there and you can get some GR8 deals now dayz. I get 18.5 to 19.5 with my 3.9 '00 Dakota and I only monitor each tank just to keep an eye out for any change that will tell me we have a problem.
When I bought this sweetheart, I felt I was getting the best look'n, best handling, best riding and in many cases the probably best mileage there is for a mid sized American made pickup on the market,, butI resolved myself to the fact it definately wouldn't be worth my while to waste fuel just to see if it can (or will) outrun an S-10, Jimmy or Ranger. Honestly, who really cares if it will, or if it won't ? If a guy (or gal) want to go jam'n up and down the boulevard look'n for a race, go get yourself a Hemi - stuff it in your Dakota, tough up the trans and drive train, forget about mileage and go blow the doors off the competition. You'll still be driving around in the finest looking, best riding, easiest driving American made pick up out there. 2nd Gen Dakota's are the "stuff" !!

 

Solid first post buddy but... I can tell you that there are some mods out there that if used properly can increase your mileage.

 

I do have a Hard tonneau cover a drop in K&N filter and a Magna Flow muffler. All fluids are synthetic including rear end and trans. The best things you can do for mileage is keep it in top tune and keep the tires aired to the manufacturers specs. The best mileage I've ever gotten was 21.7-all interstate driving 70-75 mph. I think the 3.9 V/6 does better there rather than lugging around 55-60. Also the wing I've got on the back keeps the rear end planted! (sarcasm)

 

I have had two Dakotas both with the v-6. One 4x4 and one 4x2. Both had the 3.9. one was a 99 and the other was a 2000. 14 mpg is not normal highway or city unless you are towing or carrying a load. I regularly got 18mpg in the 2000 4x4 that had dual flowmaster exhaust. I bought it that way in 2000. It was lifted 3" with 33" tires.
I now drive the 1999 4x2 with unknown mileage and no cat. I bought it that way in 2019. I regularly got 20 mpg until recently it dropped to 14. After reading these comments it dawned on me the 02 sensor is the culprit. I had a grand am running a bad o2 sensor. Bone stock on everything it ran like crap with black exhaust ever time you stepped on it. Two shops diagnosed it. One wanted $900 to do the computer and another said it was the digital egr valve. It was neither. I put a snap on scan tool on it and it was reading lean while there was black exhaust pouring out the back. Obviously the o2 sensor. Replaced that $60 part and it was good to go.
I would recommend replacing the 02 sensor and go from there

 

14 year old thread??