Jeep: second oxygen sensor

I own a Jeep Cherokee Sport 1998, 6 cylinder in-line engine, automatic, with
about 50K miles on it. I am not happy with the fuel economy of this car. I
am constantly getting about 10 MPG in the city stop and go driving (I expect
about 15MPG), and also the idle (750rpm) is a little bit rough for the 6
cylinder engine. Replaced the ignition cap and rotor, spark plugs and wires,
added the oil conditioner, constantly add different fuel injector cleaners,
sprayed the carb cleaner on throttle plate and inside (without removing the
throttle body). No big change.



There are two oxygen sensors in the car (one before the catalytic converter
and one after). The first one seems to be OK (the reading alternates between
0.2V and 0.8V), but the second is probably bad as sometimes it shows
constant 0.8V and sometimes alternates between 0.12V and 0.45V. Could
somebody tell me if the second oxygen sensor (the one after the catalytic
converter) is in the fuel loop at all and responsible for fuel economy, or
it only triggers the dead catalytic converter event?



Thank you.

Can't say for sure but from a general article, it would seem that the 2nd
one only figgers in the warning system ... The OBD II system is designed to
monitor the emissions performance of the engine. This includes keeping an
eye on anything that might cause emissions to increase. The OBD II system
compares the oxygen level readings of the O2 sensors before and after the
converter to see if the converter is reducing the pollutants in the exhaust.
If it sees little or no change in oxygen level readings, it means the
converter is not working properly. This will cause the Malfunction Indicator
Lamp (MIL) to come on.
 
converter 
between