I am planning on chaging the geras in my 99 GT, if I modify this in my mustang, is there anything else that I have do to it, because I know that sometimes when you modify one thing you have to also modify another, is this the case with the gears?

you'll most likely have to change the gear for your speedometer. i haven't done it yet, but that's what i'm told. i'll let you know for sure in a month when i do mine. i'm not sure if the gearing makes a differenc.. i'm putting 3.25 in mine. 3.55 if i can keep it of the freeway! kaj750