changing a speedometer from KM to Miles?

[pistolkeith]

Hi fellow rust bucket lovers, I've found a new set of gauges for my 85 KC 720 with factory tacho, only issue is its in KM with no Miles option, can i simply replace the backing from my current one(with miles on it) and put it into the new one? or will the needle read incorrect? also, even though my specific truck wasnt optioned for the factory tacho will i have the correct plugins already in my loom(?) or will i need to have that all installed?

any help muchly appreciated