To clear up any confusion, JSPec cars came with the same speedo face (the clock in the dash) as the UK cars, but it was labelled "km/h" instead of "mph".
To convert the SPEEDO CLOCK (i.e. not the speed SIGNAL), the cheapest & crappiest way to do that is to put a NON-OEM mph overlay (a "110mph speedo") on top of the km/h speedo. Hence 110mph ends up being roughly where 180 kp/h was; these are roughly the same speeds. Obviously this is ok, but means if you ever delimit the car and want do do more than the factory limiter, your speedo needle will want to keep going past that point and at that point you won't have a clue what speed you're doing and you risk damaging the speedo as you clatter the needle into the other side of the 0mph stop the needle rests on when you're not moving.
The better way of "converting" the speedo, is to convert the speed SIGNAL, not the speedo face. By fitting a decent speed signal converter, this little box of tricks will put the original speed signal through a kph/mph multiplication, chucking a mph signal out the other side. You connect THAT signal to the speedo face, cruise control and power steering, and if you bought an all-in-one delimiter, you also put the "clamped" signal to pin2 IIRC of the main ECU. The only thing left to do is turn your "km/h" CLOCKS to read "m/h"; how? Cover up the "k". Job done.
Sorry to those who already knew all this, but there is always a lot of misunderstanding about what a conversion involves.
http://mkivsupra.net/vbb/showthread.php?112141-How-To-Install-a-Speedo-Convertor-and-Delimiter