Speedo cable replacement?

If the vehicle in question is the one in your avatar, there is no need to break into the oiling system of the transmission. The cable unthreads from the outside of the gear housing. If it's a '65 or older car, then yeah, you're going to lose some fluid. Jacking the rear of the car high in the air and to the right side will help. Like OwdKasd mentioned, it's usually the speedo head that freezes up and snaps the cable so make sure it turns.