[Orange97GTVert]

I doubt you want a granny gear in a Mustang, I would get a T-5. A granny gear 4 speed would essentially be a 3 speed, unless you use the granny in bad winter weather or something. There would be no performance reason to use that, in fact, unless it is trashed, whatever you have now would probably perform better. The granny gear is a really low gear made for pulling heavy loads from a dead standstill, you find them in things like dumptrucks.