The mechanism inside is magnetically inducted, there is no adjustment. Well, you could move stuff around a little, but you could also make it a lot worse or even ruin the speedo. Just gotta live with it and figure out what your actual speeds are and remember them as you go an indicated 85 through town.
The local police here in Canandaigua will be glad to tell you on the ticket.
We also have a lot of those radar signs around that tell you how fast you are going, when ever they set them up I go back and forth to see how accurate mine is. Now I know.
