Gas gussler

I have a 97 jeep grand cherokee. The exhaust manifold cracked so I replaced it the other week and it is still sucking gas like crazy i've done run 2 tanks full through it and I figured the computer would have time to adjust. I also have downsized my tires from 31x10.50's to 30x9.50's and it got really good gas mileage and my check engine light aint coming on so I know its not the oxygen sensors and I scanned it just to double sure. And the exhaust rattles when its in reverse or even all the time. And I cant figure out what its hitting on all the hangers are still good and I dont know what to do. I have came up with a few solutions like taking the catalytic converter off because I dont know if its creating too much back pressure and if thats whats causing it to suck fuel. But I dont know if that would screw with the computer system and really cause it to suck gas. But I was gonna throw a throttle body spacer on it too to see if that would help a little. My dad put one on his dodge and it helped a little. I'm 19 and I live in S.C. And in april I am going to wyotech for their diesel program and thats alot of gas anyways for that far away but with the way its been doing lately I been putting every single little bit of money I have in my pocket into my jeep. I love jeeps. Its my first vehicle of choice. Besides a dodge. But its not getting the same gas mileage as it was a year ago when I got the jeep. Its a really good jeep I just wanna make it better. And I know how to do some mechanical stuff but when it comes to computers I havent brushed up on that stuff yet. And I wanted to get someone elses opinion on it that actually has more experience with that than myself i'm mostly self taught but I learned alot from my grandpa so I pretty much know all the old school stuff, but not so good with electronics. Do ya'll know anything that'd help or have any tricks tips or information that would help me. If ya'll do i'd really appreciate it cuz gas is getting really expensive!