I was driving on the highway yesterday and all of a sudden the speedometer goes to 90pmh when I'm going 60 or so.. I got off the interstate and it stopped at 30mph or so when I was at a complete stop.Turned the truck off when I got to where I was going, and it sits on 20mph.. any ideas? No check engine light or anything on the dash and nothing else wrong. The miles counter is still accurate though.