Over reactive speedo gauge

[IcebergRS]

Now I no the 172s are quick but when I plant my right foot and rev her up the speedo gauge will jump from say 50mph to about 120mph then jump back to what ever speed I'm doing! Is there anything causing this or do I need a new speedo cable?

 

[IcebergRS]

Anyone ?

 

[daveypower]

Could be a problem with the motor on the gauge for the speedo.