1995 Ford Escort Car won't downshift

I was given a 1995 ford escort. The gentleman who gave me the car said it had a problem with the speedometer cable that was creating interference with the transmission that was causing the tranny to not down shift. The car will eventually down shift on its own when its ready to, after maybe 2 miles or so down the road. Of Course we all know that a car should not be driven when the car is hitting the rpm's at 4000-5000 or you will blow the motor. My husband went to look at the speedometer cable and the housing that the cable sits into is all rotted out, but we both suspect that it might be the speed sensor gone in the car. Do you have any ideas before we go and start changing all kinds of parts that are not needed. And would just a bad speedometer cable make the car not downshift, could it be that simple. And by the way the speedometer does not work at all. Thanks so much