Brett the rear O2 sensor does absolutely nothing in terms of a/f adjustment. It is there SOLEY to monitor the catalyst efficiency.
Personally if it were my car,(and I didn't have the tools I do) I would borrow a good generic scan tool and monitor a few things.
I would look closely at the ECU coolant temp sensor readouts. See what its reading stone cold, on warmup, warm and hot. Look a few times. If its off what you know the actual temp to be or does not at least follow the temp guage on the dash, I would carefully check the connection/wiring to it and then, if that is good, replace the sensor. Make sure you get the one for the ECU, not the gauge(if 2g cars are that way)
on that note, also make sure that the thermostat is not stuck/sticking open or partially open as if the engine never fully warms up, it wil use more fuel. The ECU uses coolant temp as one of its variables for fuelling the car.
I would then look at the pre-cat O2 readings and see if it starts to cycle once its warmed up and also to see that it warms up fairly quickly. The thing should be cycling in a few min, even in this weather. The pattern should be a large analogue waveform(rounded peaks and valleys) that goes from close to 0v up to close to 5v each time(for the most part, some hashing/crappiness is ok). This waveform should change frequency with RPM up to when it goes out of closed loop and flatlines. If any of this is scetchy to you, replace it, but again, check the wiring and connections for corrosion, loose pins etc.
If the car is not misfiring or loosing obvious power, there is really no practical reason at this time to replace anything like plugs, wires, coil, etc. Those types of parts will cause bad fuel economy when they cause noticable misfires and/or noticable lack of power.
You could also do a compression test of you have the equipment. This can be important as no matter what else you do to the car, if the engine cylinders are not sealing, the car will not get good mileage, make good power(tho turbo cars can often mask that) or have low emissions.
And, lastly, I would have the injectors cleaned at a good place. Take them out and to a place, not have them flushed in the car. While this does work, the ultrasonic cleaners are better and they can flow test them etc. Put on a set of new O-rings etc and put them back in. Maybe don't do this last either, unless you know the injectors have been professionally cleaned in the last couple of years, or are new in the last couple of year, its worth it anyways. Its only a hun or a hun and a half.
Injectors CAN cause bad mileage without causing any misfires, If the spray pattern is more like a garden hose with your thumb over it than a plant mister, you will get shitty mileage. You will also pollute your catalyst over time, depending on how bad it is.
With a well tuned 2g I would think 500+km would not be that hard an achievement. And that is not taking anything away from Kevin, as obviously he keeps his car in good shape
I am able to get that and more out of my 1g car and it has only a 1g tank not a 2g tank.(10l less)
And, just another thing, in the winter, you get slightly less mileage due to colder temps densifying the air, allowing more fuel to be used most of the time. That and longer warmups, more idling, more alternator use(lights on more, heater fan on high, rear defrost on more etc)
Also ethanol fuels take more mass of fuel to produce the same energy, so if you are using an E15 fuel(15% ethanol), then you are getting 60% of the energy out of that 15% of the mass of fuel. Not huge, but noticable if you keep good track of your fuel economy.
