Will removing muffler rob gas mileage
Why do you want to remove the muffler . . . ?
If it’s rotted out, just go to a reputable independent muffler shop and have them weld in a new one
Rob gas mileage , No but it will get you a fine for noise and really annoy your neighbors.
No way to tell what it will do to gas mileage in advance. Just nosy, but why on earth would you want to do this?