Speedometer going crazy

A few days ago I noticed my speedometer was pinning itself as far as it could go.Now it holds steady,but its almost 20mph over than what my actual speed is.Also the odometer is putting on 3tenths of a mile less.I must keep my ez-scan plugged in to see how fast I am going.Has anyone else experienced this before?

sounds like your cluster messing up... the ez scan connects to the aldl connector? if it does then your speed sensor is good, so it has to be the cluster it self