Exhaust ???
 

straight piping a 2006 cummins, bad or good idea? chopped off right before the muffler the cat is still there, would this cause fuel economy to drop about 2-2.5 mpg?

Straight piping the truck should not cause the truck to loose fuel mileage.

What Scott said... If anything it will help it. (Not much but it should in theory.)

thats what i thought, the only other thing I did before my mpg's kicked shits was put a container of prestone diesel additive to the tank when i filled up, guess best thing to do now is to go through its current fuel and never use prestone again? unless anyone else out there has any idea what went wrong?

I straight piped my truck about 2 weeks ago. My fuel milage dropped about the same as yours. Now its about .3-.5 MPG higher than with factory exhaust.

Was the drop caused by cutting the exhaust off or was it bad fuel? I don't know but it seems it worked its self out now.

Run that tank out. Try an RBP, Stanadyne, or Power service additives. Also how heavy are you in the skinny pedal? Cuz thats a factor to. Not trying to sound like a dick

I agree with scott, I don't run any additives though and I'm very light on my pedal. (I like fuel mileage)

Scott why would both of our trucks loose fuel milage after straight piping it? Could it be possible that the computer isn't used to having less restriction and has to learn new settings?

I do keep my foot out of her, most of the time at least. I will run this tank out and try you suggestion scott.

My guess is that after you cut the pipes off and or
Deleted the muffler you were probably enkoying the new sound so much that you were unconciously pushing a little harder on the skinny pedal. But once you get used to it you naturally go back to your normal driving style.

Yep i went through that when i first bought my truck.