I haven't found a way to adjust the speedometer, but they are electronic and I have seen some gizmos that are supposed to fit in line between the sending unit and the speedo. You are supposed to be able to adjust it for tire size so that might be a solution.

As for me; I just use a ScanGaugeII. Mine is mounted above me rear view mirror so it is easy to see. As a matter of fact I have gotten so used to looking up there to read my speed that I now look above the mirror to check my speed when I am in any other car (at which point I quickly realize that I'm not in the VX and then look at the actual speedo). The ScanGauge speedo can be adjusted for tire size and shows all kinds of other helpful information at the same time.