Fuel mystery

I have a 1997 Altima with about 120k on it. I bought it used in 2000 with about 17k on it. According to the manual, it is supposed to have a 16.5 gal. Fuel tank. When I ran out of gas early on, I noticed that it took only 13.5 gal. Ever since then, the amount it takes to fill it when I intentionally let it run out of gas has gradually decreased. Not it takes about 8.5 gal.

The fuel gauge never registers more than about 3/4 tank, but then it seems to go down correctly.

My mechanic checked the sender unit and the tank (to make sure it wasn't collapsing) and couldn't find any problem.

I even brought it to him after filling it up, and he visually checked and said it was full. When I brought it to him empty, he confirmed that it was empty.

I'm getting normal gas mileage, with no leakage.

My guess is that something is causing a sensor to detect that the tank is empty when it's not, but my mechanic's visual inspection of the empty tank seems to contradict that.

Any ideas?