I have a 2003 Silverado 1500HD with the 6 liter V8. The speedometer is acting crazy. Sometimes it reads 40 to 60 MPH over actual. It even shows 40 MPH or more at a standstill. All the other dash guages/lights are working fine. The problem comes and goes, it happened the first time last week when we were having some unusually cold weather when it dipped below freezing a few days. I live on the Texas coast so freezing temps are rare around here. Any guess as to what might be wrong? Did cold weather cause it? Any help? I'm not a mechanic, but I can do a few things.