Answer:
Largely it depends upon the owner and how full they keep it but at maximum it will hold 16 gallons, the rest is up to you.
According to the 1998 Ford Taurus Owner Guide : ( 16.0 U.S. gallons )
( 18.0 gallons ) according to the 2000 Ford Taurus owner Guide
( 18.0 U.S. gallons ) in a 2003 Ford Taurus
For a 1998 Ford Taurus : ( the gas tank capacity is 16 U.S. gallons )
According to the 1998 Ford Taurus Owner Guide : ( 16.0 U.S. gallons )
According to the 2003 Ford Taurus Owner Guide : ( 18.0 U.S. gallons )
According to the 2001 Ford Taurus Owner Guide : ( 18.0 U.S. gallons )
According to the 1996 Ford Taurus Owner Guide : ( 16.0 U.S. gallons )
According to the 2000 Ford Taurus Owner Guide : ( 18.0 U.S. gallons )
According to the 1999 Ford Taurus Owner Guide : ( 16.0 U.S. gallons )
16 Gallons
18 U.S. gallons