There are actually three types that I have seen:
a) Converter - if you 'plug' it into the feed from the transfer box it muliplies the signal by 5/8(ish) to turn it into mph. Then your speedo reads in mph and so does the trip counter. The numbers on the speedo would read as mph too (you usually stick a sticker over the kph logo). This also means that the limiter now cuts in a 180 mph (rather than 180 kph).
b) De-limiter - this stops the signal from getting above the 180 reading (dunno how - I've forgotten the name of the electrical components it probably uses) thus the cut-out doesn't happen. Speedo and trip still read in kph. I guess you'd wire this in between the speedo and the ecu (unlike the above).
c) Both - does both.
I bought my converter from Abbey. It was fairly easy to wire in (there are some instructions floating around somewhere). I don't care about the de-limiter bit because if my R gets anywhere near 180mph I'd be happy
If your dash is dodgy you could try fitting a UK Sunny GTI or GTIR dash from a breakers. You'd get a clock too. I would assume that you'd still need a de-limiter or perhaps a converter. Some one on the yahoo group has done this.
