i thought i would get an extra tire and would hate to have to put a 65 size tire with 3 75 size tires. i dont want to get a hinged bumper. could you secure one to the roof with out buying one of those racks? would that effect my gas milage

If you don't have a hitch, it'll fit pretty well. With a hitch, some have managed to stuff it under there by letting most of the air out.
I don't advise putting it on the roof, it's a LOT of hassle and will put a pretty noticeable dent in your mileage.