Broken speedometer

My 2004 has a speedometer that starts at 40MPH when the vehicle is started.
As I drive it will be the wrong reading by atleast that amount. I unhooked the neg. battery cable and this fixed the problem for acouple weeks. Any idea's?

Usually caused by a stepper motor problem in the speedo head, very common.

Is this difficult to replace? I would assume it requires taking the dash apart.

Dash cluster and speedo must be removed and taken apart.