Does the speedo have “dip” switches? or is it digital (electronic) odometer and analog (needle) mph indicator?
I set all my Autometer electronic speedo’s like Paul suggests. I go to a local divided highway when the traffic is light and stop at the same mile marker every time. Then I drive the 2 miles at a constant speed and stop after two miles as Autometer directs.
If yours has dip switches on the back of the speedo, then do this with someone with their cruise control set at a constant speed. Just look out for traffic.