I just bought a 1987 Toyota pickup and the speedometer won't go down past 10 mph so when I am at idle or stopped the truck says that I am going 10 mph does anyone have any ideas of what it might be or how I could fix it? Thanks!

The older model trucks get a lot of grease coming up from the speedo cable and get into the speedo head. Once enough is in there it is contaminated and will have erratic function. I would just go and get a used head or get a new one and be done with it.
Replace the old worn out cable while you're at it.