Why is my 1999 Corolla has such low gas mileage?

[Bad_dude]

Hi Guys,
I just got my 1999 Corolla about 2 months ago. The engine is clean and spectacular as a new car. The thing is that the gas mileage is much lower than any other Corolla that I have ever owned or any of my friends' owned for that matter. I usually have to fill the tank with about 11 gallon. I get only about 273 miles out of the 11 gallon. I drive mostly freeway miles to work. I rarely ever get big time traffic. I feel like I should get more gas mileage out of the car. I have changed the transmission oil, engine oil, and all basic maintenance. I changed the spark plugs and cables. The tire inflation is proper.
What else can I do to get better gas mileage out of it? I don't floor the pedal when driving but just as gentle as I could on the pedal. Also, I just put in a bottle STP fuel system cleaning which gave me quite a bit more power. But the gas mileage is still the same, not very good.
One more question, the seatbelt where I attach the seatbelt by clicking in, is a little loose. How can I tighten it?
Thanks for any help.

[RadRacer3]

Check your 02 sensor
Change your Air Filter
If its a manual tranny, use higher gears to keep the engine speed down
don't accelerate fast

I have changed the transmission oil,

I take it this means you have an automatic transaxle. You will inherently have lower fuel economy than corolla's with manual transaxles.
when parked at a stop light or drive through or etc.... put your transmission in neutral don't leave it in drive.

I just put in a bottle STP fuel system cleaning which gave me quite a bit more power

No fuel additive will give you more power, infact you might harm your fuel system. Any power gains from off-the-shelf fuel additives are psychological.

25MPG does seem low, but I've never owned an automatic corolla. I would think 30 - 32mpg highway would be acceptable. My 95' Prizm used to get 36 highway.

Other causes:
Loss of cylinder compression
bad PCV valve
Clogged Fuel Filter/bad fuel pump

[Bad_dude]

If the O2 sensor is bad, wouldn't the check engine light might go on? By the way, where is normally the O2 sensor located? If it's not bad, is it easy remove for checking? Also, does it needs cleaning for performance improvement?
The air filter has been changed also. Some one recommended me the Flow Master which I can't find anywhere. Perhaps anone can shed some light on it?
The tranny is automatic.
The car response to the pedal pretty well and drives well all the time. I don't think the cynlinder compression is lost of any sort. It has only about 89000 miles on it.
PCV valve has been changed. The fuel filter is built into the fuel pump for the lifetime of the car. It has no symptoms of clogged fuel filter.
What else could it be?

Thanks.

[RadRacer3]

I'm not sure if the check engine light will go on or not. It's located on the down pipe just after the header, (there might be 2 of them on a 99' ?)

If it is bad, your car will run rich (programed default) so you will get less fuel economy.
You don't need to remove it for checking, either use a obdII diagnostics computer or use a volt meter and measure the response. You can't clean it.

[Bad_dude]

Originally Posted by RadRacer3

I'm not sure if the check engine light will go on or not. It's located on the down pipe just after the header, (there might be 2 of them on a 99' ?)

If it is bad, your car will run rich (programed default) so you will get less fuel economy.
You don't need to remove it for checking, either use a obdII diagnostics computer or use a volt meter and measure the response. You can't clean it.

How much and which type of code reader is best for the job? How do you know if the car runs rich?
Thanks.

[RadRacer3]

The cheapest way to check diagnostic codes is to take it to a shop, or borrow one from a friend. GM makes the TECH II which should support the "Chevy Prizm/Toyota Corolla". Autotap would be another option.

If its running rich you should be able to smell it (But thats for an experienced nose to determine) Check the voltage out of your o2 sensor when the car is running
or read this article on ow to check it http://mymiata.paladinmicro.com/Miata4-WireO2.htm,
I would first try checking it while still installed on the car.

[Bad_dude]

Is there an easy way to test the oxygen sensors? I know my car has two of them. Or if you could please describe to me how I can test it? I know I would need a voltmeter but what kind?
Thanks.
PS: I need to buy a repair manual for my 1999 Corolla, which one is the best for beginners like me?