I ran into this problem yesterday.
I left my SSR out of the garage (with the top down) for about an hour. The sun was shining on the speedometer. When I started the SSR and drove out of my driveway I was already doing 15 MPH at a near stand still.
After the dash cooled down and I pulled off on a side road to do a power down and restart, the speedometer seemed to get back to reality.
I'm assuming others that have experienced this problem and had it "fixed" had the SSR gauge cluster (including the speedometer) replaced?
Also, my 0 (zero) MPH (dead stop) actually has the needle about 2 MPH below zero now..