I have recently replaced the fuel filter and air filter. I was expecting better economy by changing the fuel filter (it was a real B*tch to change) since it was never ever changed.

My gas mileage is still the same (15 miles/Gallon). BTW, I only do city driving and rarely go on the freeway.

Maybe a general tune-up would be in order. Take it to either a Toyota dealer or a reputable Toyota repair shop and have them change the spark plugs, set the timing, etc., and have them hook it up to their analyzer and scope the engine for any kind of compression problems, etc. Then, if I were you, I would have the engine flushed and the oil/filter changed. Then I would take it out on the freeway for an hour or two and blow it out so to speak. You don't have to keep the gas pedal to the floor all the time; just run it a good high speed. See what happens. Have you changed your timing belt? They are to be changed every 60,000 miles. Also, it might not be a bad idea to have your auto transmission fluid changed. I do mine every 15,000 miles on my 92 Camry. The older they get the more we have to keep after them.

I changed the plugs not too long ago, however, I did not change the wires or test them for their resistance.

The only thing I can think of is the oxygen sensor. But in my 2VZ-FE engine it is located in a spot that looks like the intake manifold has to come off.

Toyotas are sometimes too complicated.