It may not have to do with any of the things we've discused so far...he may have busted rings for example, and that's whats he's hearing, that would affect mileage.
A bad temp sensor would make it run "too rich" for example.
Guy's will come on here and not mention the check engine light has been on forever, but it won't be mentioned because the car was still moving and they need it to run "just a little better" you know, to maybe get to work a little more.
The muffler/res may have punched a small hole in the gas tank on it's way to freedom for all we know. Honestly, if you think we're good at this with "described symtoms" try to imagine being able to actually put hands on.
We got a guy on here discribes "some" of his symtoms like the "car won't idle" but won't mention (we find this out two posts later) that he also knows the intake is sucking in coolant from when he put it all back together badly last time, then wants to blame the car!
Hey, we try to help out where we can, but It is what it is.