The car I have is a 95 wrx, and is fitted with a small white box next to the ecu made by INTEC and fitted by Global autosport( now ralliart) this box delimits the cars top speed but does not alter the mechanical function of the speedo or odometer, these still read in KM.
Having searched the bbs this would appear to be contrary to the widely held view that this is not possible, well it is!