* Check the owner's manual: This is the most reliable source.
* Look inside the fuel filler door: The gas tank capacity is sometimes printed on a sticker there.
* Check the vehicle's sticker: Often, there's an information sticker located in the driver's side doorjamb or elsewhere within the vehicle listing specifications, including fuel tank capacity.
Without more specific information about your 1989 F-150, I cannot give you a precise gas tank size.