According to my Chilton's Auto Repair Manual :
A 1991 Ford Mustang has a ( 15.4 U.S. gallon gas tank )
16.0 Gallons
20 gallons
16.0 U.S. gallons
15.7 gallons
15.4 gallons
15.7
15.5 U.S. gallons
Your gas tank will hold 16 gallons of fuel
16 gallons - according to the 2008 Ford Mustang owners manual
16 gallons
I believe that is 15.4 U.S. gallons
15.4 U.S. gallons