When I'm start my car the over drive light flashes 16 times. As I drive the rpm just keeps going up as the speed goes up. I do not see them rev down nor up when gear shifts. If I turn the over drive on or off as I'm driving there is no change in the rpm. By the time I'm going 80 the rpm is showing 4000. What can cause this? I've looked around but I'm yet to find an answer.