How many miles does the car have? There are a ton of things that could cause bad mileage so I'm just going to throw some out there: Catalytic Converter is clogged/semi clogged, O2 Sensors work but are weak (you can check this with a voltmeter) fuel/air filters, EGR, PCV valve clogged, throttle body is dirty, needs new fluids (you claimed you changed them all, Did you really? I mean ALL OF THEM?), and then the ideas mentioned above like dragging caliper, etc. It's hard to know because we don't know how many miles it has as for all we know it needs a major service done to it which would readjust the valves, spark plugs+wires, etc.
I know the sensors start responding to O2 stream changes slower over time until they just fail. Is that what you mean?
Only in SFI does the slower response time usually lead to worse mileage. (In case you wanted to know, SFI has the fastest response time to O2 changes, TBI and MPI aren't as fast so the mixtures don't get thrown off as easily)
"O2 Sensors work but are weak"
When an O2 sensor gets old, it reads leaner, so the ecu will add fuel to compensate. It will still cycle the O2 volts and the ecu thinks it's working right.
One of the things I look for to determine if the O2 sensor is old is to look at the wire coming out of the sensor. If the covering on the wire is soft and rubbery it might be newer. If the coating on the wire looks like dry woven cloth with frayed ends, that's an old sensor. This isn't a definitive test of the sensors function, it's just one way of knowing relative age by simply looking at it.
If it looks old, the second thing I'd check is the color of the spark plugs. If the engine was running rich it will show on the plug tips. For reference, see http://www.verrill.com/moto/sellingg...colorchart.htm
If it's running rich it will look similar to the 'carbon fouled' plug, although it may not be as bad as that pic.
well i think part of the reason is that i work 12 minutes away, and eveywhere i go its a 5 min drive or less, and that 12 minutes to work is the longest that my car operates at a given time, and even after 12 minutes my temp needle isnt at the "normal" operating temperature yet.
once in a long while I hit the freeway, thats where the 30% highway miles came from. the car has 160k miles