ok my car is 289,000 plus miles on it and i think im having some fuel problems.

I have a 90 maxima (engine was rebuilt atleast once if not more)

i think i am having gas problems but dont know.
Its an automatic .

i put in 20$ worth of gas thats like 10 gal

the stick was on 3 quarters now the stick is bettween half and 1 quarter. and the meter says i drove 80 miles.

Some times i smell gas and some times i guess smoky gas when im driving (dont think its th cars near me)

Now how many mpg should i be getting?
how do i test out what im getting? fill it up and drive till the car stops?


if there is a fuel related problem what is it usually?

like i remember in the winter some times witht he heater when it was on i could smell gas.