Silverado MPG mileage flowmaster

ok i was getting about 15-17 around town...and 19-20 on highway with my 2000 Silverado 5.3L. I recently got a Flowmaster 40 series muffler, and ive cleaned the fuel system, throttle body, MAF. And since all that...im getting about 11-12 now...all together. It sucks. What do yall think it is?
Thanks
This is the exact opposite of whats suppose to happen when any of those things are done.

ok i was getting about 15-17 around town...and 19-20 on highway with my 2000 Silverado 5.3L. I recently got a Flowmaster 40 series muffler, and ive cleaned the fuel system, throttle body, MAF. And since all that...im getting about 11-12 now...all together. It sucks. What do yall think it is?
Thanks
This is the exact opposite of whats suppose to happen when any of those things are done.

this is what happens when you de-tune the system...
prior to you spending all that cash your MPG is what I get on my 2000 5.3L 4X4 LT ....19-20mpg hwy...

this is what is expected to occur with these vehicles...

everyone that ive talked to who had a silverado and put a new exhaust on...never say anything about how they lost MPG. Is there a way to regain it other than putting the stock system back on?

are you sure its ur exhaust like did it happen right after it, and also another thing to think about if u do or not cause i know i did that made my gas mileage suck till i realized i was doing it is when u get a new exhaust its louder so u might have the tendencie to hit the gas more or accel harder or stay in it longer so hints ur gas mileage goes down lol but all in all i put on custom true dual 3inch striaght pipe on my 2000 and didnt effect gas mileage minus me making it effect it

yea thats whats wierd...it didnt happen right after i got it installed, i started guzzlin gas like 2 weeks later. I think me hittin the gas harder had something to do with it cause it did slightly increase take-off + the sound of it lol. But im really not floorin it or anything, and went through and cleaned the throttle body, and MAF, and got my fuel system professionally cleaned. Its only picked up maybe 1mpg from that. I dont know what else to think of, to check/change. I had autozone test it, no codes came up. I was possibly wondering if my Oxygen Sensors were bad, but it didnt throw a code.
HELP!!! lol

lol i really dont know cause o2 sensors is what i was going to say but if u dont have a code then idk, and trust me codes pop quick with those lol, have a problem with one of mine now im tryin to hunt down lol, but yeah idk what else it could be if no codes are goin off or anything, fuel pump going bad if that can do it u think? thats my only other guess