Hey Scotty sir, i went ahead and replaced the engine in my 2010 Honda CRV (RE2) with the R20A engine with AT transmission and so far so good, except i recently noticed a drop in gas mileage, initially the cat broke and was ringing in the exhaust and i had to dismantle the exhaust to remove the pieces but the gas mileage still did not improve, i then replaced the cat with a good used on and reused the sensors. 

When i check the OBD data i see that i run at around negative 3 to negative 5 short term fuel trim at idle with some minor blimps to 10 but only for split seconds but my tool does not show the LTFTs. I have yet to compare the gas mileage post cat change but in short trips it seems to not be that much better. What could be the problem? I recently changed my air filter the coils have been tested and are good, the plugs are brown in color with almost no sooty deposits. I remember that i once changed the fuel pump to an after market one could this be the issue? My fuel injectors are remanufactured once and recent so i'm sure that's not the problem. I also did a vacuum test and it was within range from idle to 3000 rpms. I ran the car without the bank 1 o2 sensor and found the bank 2 post cat sensorreadings were at about 0.8v meaning the car is indeed running rich.

PS: Used to get 24MPG with low traffic now i'm at 17MPG and used t get 30MPG on the highway but now its 25 MPG

Should I get new MAF, MAP and O2 sensors or could the pump be the issue?