IF you knew exactly where everything is inside, you could probably drill a small, pluggable hole in the case and give it a tweak.
The common adjustment is to vary the tnsion on the return spring. On ones that have to be very accurate such as police ones, they also adjust the magnet airgap so that it is more liner.
On my bike, the speedometer seems to track accurately, but it is high by 10 MPH from about 30 on. I'm not sure what to do with that other than taking it all apart and rotating the dial. If I tried to set the spring tension, it would only be accurate at one point, and read under or over depending on whether I went faster or slower.
