On my 2009 chevy impala the miles per hour hand is jumping around. Some times it jumps up to around a 100 mph while sitting still in drive at idle. This effects the engine also. This started about a week before the shifting started to sound like it was shifting up and down while driving at a low speed around town.
Hope someone can enlighten me as what could be the problem.
Thanks in advance

 

Could this be the vehicle speed sensor (VSS) or a bad motor for the speedo?

 

Yep. If it's affecting rpm's IMO it is the speed sensor.

 

I tried to get to the speed sensor but no luck. Just couldn't see how it was attached. It looked like there was a line clipped to the threads of the sensor and I have no idea how the line came loose. Other than that I believe I could have changed out the sensor. So I carried the car to my "trusted?" car mechanic and they had it repaired in one day at $275. I think the part was about $35 and the rest was labor and shop things. They gave me a 12 month warranty.
By the way, I have no idea what that line with the clip was. It didn't look like it was part of the sensor. It just looked like they anchored another line to the stud that held the sensor in place.

 

I am having the Same issue. Was wondering if you have a schematic?

 

No schematic. Just gave up on it and let the mechanic repair it.

 

I've got the same problem - speedo jumps up when in drive and rpms reduce causing stalls at intersections or changing into reverse. Have Changed VSS twice (speed sensor) to no effect. Seems to be worse in hotter temperatures and often gets better on longer journeys