As far as fuel economy goes--check the fuel injectors and cold start
injectors for leaks. Make sure your O2 sensor is actually connected.
If it is, check your mixture. Check the coolant temp sensors and
the ignition advance, etc. If I drive steadily (which is rare), I can
squeeze about 20 mpg city and 26 mpg highway. Usually I get about 18
city and 23 highway (cruising at 80 mph).
This may sound odd, but what does the exhaust smell like? If it is
running considerably rich, you should be able to smell it.
I'm short on time right now, but I know how to check an O2 sensor
with a digital VOM.
Joe