speedometer problems

I noticed when going through construction that has the radar billboards my speedometer is 3 MPH faster than reported speed. This is both northbound and southbound, at least 4 different times. Any idea of what it may be before I take her in?

OK. Pressure washed my truck. Probably got a little carried away with the water. Long story short, ended up with about a dozen different codes and a truck that wouldn't start. Opened up all the connectors that I could find under the hood and used compressed air to dry them out. Used electrical...