I was driving my mother in laws 2001 Impala and the speedometer started acting up. It starts out working fine and after driving for a little while it starts acting up. You will be driving along at less say 55mph and you slow way down it might stay at 45mph when you know you are doing about 20mph so when you speed up to the speed of traffic (about 55mph) the speedo will say 75mph. So as you drive down the highway as you slow down and speed up in traffic . The next thing you know the speedo says 120mph and people are flying past me. To get it back to zero at a complete stop I have to keep turning the car on and off each time the speedo will drop 10mph with each restart.