Would be easier to just make paper where you write mph-kph side-by-side (example in 5 kph increments)
But for tech people, paper is not acceptable :P
It is not so hard to make some custom functions there. that 64bit math makes thing little harder though.
basically you make function like this (probaby dont even need 64bit math)
unsigned long instantkph()
{
unsigned long kph = instantmph(); // 60.54 mph is 60540
kph = kph / 1.609344; // result should be 37617 so it means 37.61 km/h
return kph;
}
if you want, you can make extra screen for this purpose (not hard at all), but easier at first is to use custom screen - replace there some slot with instantkph()