I noticed today that my speedometer is acting strange. It works but it moves very slowly now. The car is accelerating much faster than the speedometer is rising and when I come to a stop the speedometer doesn't quickly drop back to zero. It takes several seconds to slowly drop back down. So basically I can be sitting still at a stop light and my speedometer will still say 35mph but after a bit it it finally makes it's way back to zero. What's wrong with it?