I didn't get an owners manual when I bought my car... from here I've summed up that I should be using 87 in the car since it's stock, but I was curious what size the gas tank is? Its a 2003 GT. Thanks.
if you go to ford's site and signup for a 'my ford' (or whatever its called) account you can download a .PDF version of all the various books that originally came with the car.