stock s10

Hey all, This is something that has bugged me since I got my truck. I've got a 2wd '96 s-10 w/4.3L V6 w/AT. Anyway my speedo has been off by +5 mph since the day I got the truck(if speedo said I was doing 60mph I was actually doing 65mph). I've kind of half looked into it but never did anything cause I knew about the problem and adjusted by adding to my viewed speed. So here's the thing is this actually a speedo problem or a speed sensor problem? Any info as to how to fix this to where speedo reads accurately is appreciated.