I was running ~15 to almost 16 on the overhead constantly, with stock setup, 4x4 OR.

put on a tonneau, and showed 18~20 a few times.

put on the straight pipe and now its round 16~17 for most part.

i've constantly resetting, so not sure those were good numbers, all this done in one week.

should i have left the muff on?

too loud for me, but wife likes it.

 

I put a straight pipe on my truck a few months ago. It's milage is the same or better, depending on the day and where i filled up. It was the best thing i did to it. LOUD AND PROUD

 

My milage has stayed around 20 mpg if you were wondering. That is both on the computer and by my own calculations.

 

When you switch from stock muffler to straight pipe, you will have a tendency to push it a little harder so you can hear it. This will eliminate any increase in mileage. after a few months (when you get use to it) milage should pick up.

 

I agree with Roton about "wanting" to hear it so you push it a little. I just put the programmer on mine and can't keep my foot out of it. When I set the cruise at 70 going to work I hit 22-23 mpg. I noticed very little if anything when I pulled the muffler off. I left it off because I tow and less restriction lowers the EGT's and that's more important to me. All I used to hear was the engine. Now, all I hear is exhaust. I always drive with the windows up and that makes a huge difference to windows down. I would roll them down on occassion to listen to the turbo. On thing to note is that I have a cat on and that muffles the sound as well.