Why is my car wasting a lot of gas? is this normal? 2001 Pontiac Grand Am GT?

I own a 2001 Pontiac Grand Am GT and been having it for 6 month and i realized the gas isn't good and it s not a gas leakage i double checked. My mechanic said could be the fuel pump, keyword MIGHT. I am not going to risk it i don t want to spend my money on that and not be it since i did it with the... show more I own a 2001 Pontiac Grand Am GT and been having it for 6 month and i realized the gas isn't good and it s not a gas leakage i double checked. My mechanic said could be the fuel pump, keyword MIGHT. I am not going to risk it i don t want to spend my money on that and not be it since i did it with the suspensions since it was squeaking (still haven't fixed the squeaking noise) but i notice because i put in $15 of gas when it's close to empty and it goes to the middle then when i drive it for 8 miles it goes from the middle to quarter to empty and i know that is not normal since my truck doesn't do that. A Grand Am GT has 21 mpg in city and 32 in highway. What else can cause this other than fuel pump? or is that normal for 21 mpg? How can i be sure if it's a problem without having to spend tons on a part that's not it. I am not spending $300 for a new fuel pump if i am not too sure about it. Thank you!
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