I thought that I had seen a device, somewhere, sometime, (I know, real specific) that you could put in line behind the dash cluster that would allow you to recalibrate for tire size. I would have to search the Googles to see if I could find it again.
As far as I know there are no programers available for the VX, and don't know if there are any for Isuzu in general. Like Rowhard, I have a ScanGaugeII that I use for my speedo now.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless