Before you smack your forehead and groan "not another mileage thread", bear with me as I've tried to narrow down my problem.

Recently deleted my precats on my 02 auto SE, and as expected P0138. Now I'm not due for smog for another year, so I'll worry about installing an O2 sim (or spacer) later. I had also replaced my Bank 1 primary 02 sensor "just in case" because the precat on that bank was toast, so i figure it could have been a bit lazy.

However, I'm finding that I'm getting approx 14mpg vs ~22 that I got the very day before i started taking stuff apart. (calculated by dividing miles from gallons). I'm hesitantly ruling out the usual suspects - same driving habits, tire pressure good, new oil, new plugs, MAF has been cleaned, ran a bottle of injector cleaner..

From what I've read, the secondary O2s aren't used for fuel control, but someting i did must have affected my fuel economy.

A few theories

Theory #1. no precats means less heat meaning it takes longer to warm up, meaning car is in open loop longer

Theory #2. one of my primaries are lazy or I didn't install the right sensor for bank 1 sensor 1. But shouldn't the ECU throw an SES light if this is the case?

Theory #3. i have an exhaust leak that developed after putting everything together, and that's killing my mileage

Theory #4. For some reason, when I put all the exhaust components back on, i did it such a way that the one or more (or all) of the 02 sensors aren't properly grounded? I used to have a turbo eclipse that made a big deal if the exhaust ground strap wasn't attached.. but the Max doesn't have one. Maybe its time for a ground wire kit? Again, shouldn't ecu throw an SES light?

Theory #5. With a P0138, ecu gets confused and starts dumping fuel? Install O2 sim and problem goes away? I know, I'm struggling here.

and finally

Theory #6: Maybe all the fuel trims are fubar? I notice that my little mpg meter gave up the ghost like 5k miles ago and is permanently stuck at 21.1. Perhaps if I just unplug the battery overnight, the ECU can relearn and things go back to normal?

Thanks in advance guys.. you've all been super helpful and supportive!