I have a Garmin GP45 GPS that i use for my speedo on the highway. For about the same money as you would spend on the compensating unit you can have a lot more info. Mine track-plots, gives speed and direction, etc. It's not permanent to the truck, however i carry it in the jon boat too.
I run 315/75-16 BFG's so the speedo is off about 12-15%.
For me it is a good option to the recalibration problem of oversize tires.
Tom