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

It could be just your city driving. If you are forced to stop frequently and set and idle, it hurts your mileage. Short trips hurt too, the engine has to have a richer mixture when it is cold. Do you have a temperature gage? If the thermostat is stuck open, it would show a low temperature which would hurt the mileage. Some of the sensors could be bad, but they are hard to check without hooking the car up to a computer that reads out everything. A lighter oil may help too. Most cars in America do fine fine on 5W-30, but for Europe, the same engine may run 15W-50. No good answer why. The lighter oils also make for easier cold starts.

Thanks for your answer.

I do have the temperature gauge, and the engine heats up fine and remains properly heated. I use Mobil 1 10W40 oil, was afraid to get anything lighter because the car is burning some oil - around a quarter of a gallon on 1250 miles. By the way, is that too much oil burned?

On a side note, I've heard some rumors about some sort of magnetic gadget that you put on your fuel delivery line and it does some sort of good to your mileage. I must say it sounds like some sort of a bad joke, but does anyone have any insight on this?

Kresho

That oil comsumption sounds high, but many engines use that much.

Our government has tested hundreds of gas saving gagets and would likely mandate any that worked. They are still looking.

Check your tire size. If anything, larger tires should give better milage, but will definitely reduce the distance the odometer records. If you ever drive on a highway with the distance marked, compare it to your odometer.

I'll do that. Thank you for your answers once again.

Kresho