Post-cat sensor is not going to help your fuel economy, unless it is defective......as in a Check Engine Light (CEL) lit with a code that tells you that it is bad.
The post-cat (aka downstream sensor) ONLY monitors the catalytic converter to make sure that it is working.
The pre-cat (upstream sensors) are what the computer (aka ECM or PCM) uses to control the air / fuel ratio......these DO have an impact on fuel economy.
As a oxygen sensor ages, the switching speed slows down.
So, in theory replacement with new sensors can improve fuel economy, provinding that the old ones have significantly slowed down.
If you have 2 upstream sensors......1 for each bank of cylinders on a 'V' type motor, then I would recommend that the oxygen sensors both be replaced "as a pair" so that you have the same switching charactoristics for both sensors.
HOWEVER, I did replace both upstream sensors on my '96 3.8L windstar at well over 150K miles.....some slight coolant stain on 1.....repaired the coolant leak....replaced the sensors using OEM motorcraft brand.
I did not see any performance or fuel economy gain......although the motor seemed to be running just fine before and after the change.
The theory in using OEM vs aftermarket.....even though Bosch, Denso, or NKG may have made the sensor......the sensor was made to OEM specifications.
Now, if the OEM states to use Densor, NKG, or Bosch, and the number number on the part is the same as the aftermarket.....then there is likely no difference.
