The question you ask is too involved for a proper answer. You may want to go to a car parts store, or book store, and get a repair manual for your car. They cost about $16.00 Or, go to a Public Library.
17.50 Gallons
17 gallons
18 gallons
17.5
17 gallons
On top of gas tank.
Depends on the year which you fail to list.
Approx. 17.5 gal
12 gallons.
Fuel pump is located inside the gas tank, you need to remove the fuel tank from the car in order to have access to the fuel pump.
My Chilton's Auto Repair Manual shows : For a 1991 Buick Century : ( 15.7 U.S. gallon gas tank )
inthe gas tank john