mine holds 15 to 15.5 in the 18 gal tank....
thats why when I first bought the car and ran out (gauge wasnt working) and I filled it with 15 gals...I "Knew" someone had put the wrong tank on it ,and as I replaced it with the correct tank, I STILL only got 15 gals in the 18 gal tank!
live and learn...

but I do remember filling up in 69 when we had a gas price war here and it was $0.19.9/gal that I paid just about $3.00 for a fill up in my 68 = 15 gals!