I just got done takin my cat and flowmaster mufler off and putin a glass pack in with dual exhaust. How much better gas milage am i gonna get now??

 

No one can say exactly. It could go up, and it could go down. Only one that can tell is you. It's all about your driving style.

I do know mine went up when I cut the cats and put a single straight pipe on it. But at the same time it went down because I liked how it sounded so much, every time I passed someone, I got on it and let it drop a gear. :p

 

Unlikely that it will even change...

 

You are beating a dead horse with mileage. What we get is all we will get. There is no better mileage we can get due to the lack of actual petroleum in our gas.

 

If the cat was ok and the muffler wasn't damaged there should be no change or maybe worse depending on the core diameter of the glass pack.
YMMV
rikard