I've checked the air filter and it seems clean, the gas cap and tank
have been replaced, and I keep the tires at 32psi (221kpa). What else
could I do to help out, and how much would it cost?

I know this question gets asked from time to time, but was wondering if
I might be able to get some more specific information.

I read your second post, and you might want to try reducing your speed a
bit.  Try 110km/h (68mph).  Also, go easy when taking off from a stop if
you tend to floor it.

Assuming 32psi is the Honda-specified pressure for your tires, are they
the correct size?

How are the spark plugs, wires, distributor cap and rotor?  Are they OEM?

A UK gal is about 1.2 US gals, so 31/1.2 = 25.9
and 34 / 1.2 =  28.3

so 25.9 to 26
and 28.3 to 29

For his city mileage:

  100km / 9.1 l
= (100 x 0.621) miles / (9.1 x 0.264) US gallons
= 62.1 miles / 2.40 US gallons
= 25.88 miles per US gallon

And the OP said 26mpg.

I'm pretty sure your 90 Civic Wagon has the same engine
(code #D15B2) as my 91 Civic LX sedan. Your mileage
definitely seems low.

Try some of the tips at
http://home.earthlink.net/~honda.lioness/id11.html .

If the PCV valve has never been changed before, I'd pay
particular attention to it, per the site above. An update on
what you find would be welcome.

Next, I echo what High Tech Misfit said about the plugs,
wires, distributor cap, and rotor.

Then I'd start replacing/checking the other things I list at
the site above.

Do not coast on hills. Keep the car in gear. The engine
control system is actually less efficient, gas-wise, when
coasting.

Although the weather is becoming colder here, those figures are about
normal through the summer and the fall; as it gets further into winter
here, I usually see my mileage drop by an additional 1-2mpg. I keep the
car in gear all the time and never coast.

The car, I'm pretty sure, has a 1.5L D15B2 engine in it. The mileage on
the engine is about 265,000km or 164,663miles. The spark plugs, wires,
fuel filter, air pilter, pcv valve, distributor cap, and distributor
rotor were all changed at 205,000km; the spark plugs changed again
(NGKs) at 250,000km along with the oxygen sensor. Perhaps it's time to
start looking at some of those in more detail (if so, which?). The
coolant system just had a flush as well.

1. check or replace the coolant sensor.  if it's not sending accurate
information, the ecu will inject excess fuel.  you get a similar result
  if the coolant level is insufficient.  to be sure of coolant, remove
the radiator cap, do NOT rely on the fluid level in the expansion bottle.

2. be certain the ignition system is ok.  apart from ensuring all the
usual suspects are in good condition, i can report that performance and
therefore mileage can be substantially improved if an ailing ignition
condenser is replaced.  also make sure the ignition timing is correct.

3. ensure the injection system is ok.  run injector cleaner through the
system.  a leaky injector can dump a lot of gas.

I think it's at
34 51' 55" N, 136 31' 40" E

The Honda plant in Marysville is at
40 16' 47" N, 83 30' 47 W

Just cut and paste into the google earth search window.

I know you said the air filter looks okay, but it is due for
replacement about every 50k km, and it's cheap and easy to
replace, so I'd replace it.

Do you have a maintenance schedule for this car? It's in the
owner's manual but I also know some web sites that have it.

Were all the new parts you list above OEM?

Add a bottle of a fuel injector cleaner to the gas tank, per
the bottle's directions. I don't think that's critical; I
only started doing it a year or so ago with my 91 Civic, but
plenty here think it's a good practice.

Does the power (that is, acceleration) seem okay?