* Your owner's manual: This is the most reliable source.
* The fuel filler door: The capacity might be printed on a sticker inside the fuel filler door.
* Your vehicle's identification number (VIN): A dealer or online VIN decoder might be able to tell you based on the VIN.
Without knowing the specific trim level (e.g., XL, XLT, etc.) and body style (regular cab, extended cab, SuperCab) of your 1998 Ford Ranger, it's impossible to give a definitive answer.