I noticed today that my speedo said I doing 70 mph went it felt like 50 mph and I was moving with the flow of traffic. When I came to a stop at a light the speedo needle did not zero out but stayed at about 20 mph +/-.
Anyone know what the cause might be?
Thanks

 

More than likely its the stepper motor in the speedometer going out, you can either replace the stepper motors, would recommend replacing them all if your going to do one, since they'll all fail eventually. Or get a replacement gauge cluster.

 

Is it possible the "stepper motor" is bad or going bad? I know my wife's Trailblazer has that problem so I know GM got a hold of some crappy stepper motors but I wasn't aware that it affected any model year Monte Carlo.

Try this: Disconnect the negative battery terminal and see if it 0's out. If it does and then acts crazy again after a day or two I would have to say the stepper motor is bad. That is the only way I am able to fix my wife's in the Trailblazer is to disconnect the negative battery terminal which would 0 out the speedometer. Unfortunately the "fix" would only last a day or two.

If its the stepper motor you can buy the "new" ones on Ebay cheap and with a little soldering knowledge you can probably fix it.