To reset your speedo is no big problem. I wrote about this before but here goes again.

Determine how much your speedo is off by either following someone who can tell you exactly how fast they are going, or, as I do, use construction site radar units that have big flashing screens to tell you you are going too fast and that the fines are double in a construction zone. I don't know how accurate these are but I think they are good enough.

Remove your speedo from the dash.

Rotate the needle up to say 60 mph. If the speedo is 15 mph fast (you are actually going 45 mph when it reads 60 mph) then hold the shaft that holds the needle and rotate the needle back to 45 mph.

You can do this by either holding the aluminum cup wheel behind the speedo face (wiggle the needle and you will see the cup move), or by grabbing the shaft under the needle with some thin needle nose pliers. Holding the cup is better because you can't really tell if the shaft is slipping by holding it.