Andres,
One thing has to be taken into consideration.
Yes, the speedos are different :? . Not so much between KPH and miles (with the exception of odometer) but especially with regard to the rear end gear ratios of the cars.
I.e. there was/is each an individual speedo for the 1:3.75, 1:3.69, 1:3.92 and 1:4.08 rear ends. This makes a total of eight different speedos (KPH or miles)for our beloved cars. Most of those (with the exception of the 1:3.75 one) are shared with the Coupés.
You can "convert" the one into the other (I got this done once), but this has to be done by one of the specialised shops listed below since they have the other little gears necessary for the conversion. The speedos are marked at their backside with a number like "1,96"; in Germany we call that "Wegdrehzahl"; don't know the correct English term for that (probably gear ratio of the speedo or so).
My 2 cents: check back with Dan, he can certainly help you out.
Good luck!
Achim