Nissan Altima: 10 miles/gallon gas in local driving

Have you gotten any CEL's(check engine lights)? If I were to throw out a preliminary "guess", I woudl say it may be a Throttle Position sensor failure. If it is stuck at wide open, it's possible. If I'm not mistaken, which could be the case, you should be seeing a rich condition in such a situation. Check your plugs again and see if they are showing any fouling.

But my Engine light not turned on yet.
I went to a mechanic today, he could not find the second oxygen sensor on the bottom of the car. But I was told by several auto stores that Nissan 94 has two oxygen sensors. It is so strange.

The second O2 sensor is right behind the Catalytic converter. The first is most commonly found at the lower side of the exhaust manifold. Under the car, look for the braided canister looking part of the exhaust, a short way back from there will be the Catalytic converter, and almost immediately behind the Cat will be the after-cat O2. It will almost be staring you in the face if you come in at mid point from the passenger side. If it is a bad O2 sensor, you should be getting an engine light at some point.