My Honda Civic 91 DX has bad gas mileage - about 21MPG. What could be the cause of it?

I know underinflated tires can cause it, but my tires are properly inflated (as Service Manual states).
I have the following items all new, so I should remove them from the list of possible causes: oxygen sensor, fuel filter, distributor cap and rotor, spark plugs, spark plug wires, air filter, engine oil and filter.
I also know that my catalytic converter is fine.

I drive my car in the city only, no freeways at all. I try to drive it as economically as possible, so no racing for me at all. I use 95 octane gas (we only have 95 and 98 octane gas here in Croatia). I'm a mathematician so I'm sure I calculated my mileage correct .

Some guy here told me it could be some sort of fault in the fuel injection system. What can I do to check it?

Thanks... Kresho