Relatively speaking, deleting the muffler shouldn't have an effect on gas mileage. Now, if you have pulled the cat....that's a different ball game. Do you rev it at lights? That has a large impact on gas mileage.....

 

 

What year is your truck? Still have the pre-cat O2 sensor?

 

 

 

Exactly what I was going to say.

 

 

So you're not really straight piped? Way I think of straight pipes are manifold to pipe down each side with maybe an x or H pipe for resonance but true duals.

If you have factory y-pipe and cat with just no muffler, I would suspect something else is causing your mpgs. I have Magniflow ypipe with glass pack and my power went up and mpgs stayed 12-13 city.

 

Unless your cats are plugged (literally) up they offer very little resistance. If your cats are unplugged (electrically) your system (OBD2) will stay in open loop mode and uses the preset maps in the computer. When these maps are used, it throws more fuel into the mix to keep it running smoothy until it gets warmed up and other parameters are met. Like a choke on carburetor, keeps it running rich until warmed up and uses a lot of gas. If ran this way (OBD2) for extended periods it will clog the cats reducing power. Haven't researched, if you can cut the aftercat O2 sensor out of the computer parameters then running without cats is no problem and will go into closed loop mode. It'll just pollute a bit more.