Whats wrong if when you have your mark vii in neutrual or in park and rev it up and the speedomiter goes up in speed and the odomiter clicks up in miles need help?

Whats wrong if when you have your mark vii in neutrual or in park and rev it up and the speedomiter goes up in speed and the odomiter clicks up in miles need help?
Low voltage, mine did that and was bad alternator