[oldsub86]

I don't suppose they have made them in years but there used to be things called "kiloverters". It was a doodad that you plugged into the side of the transmission in the spot where the speedo cable installed. Then you plugged the speedo cable into the end of the adapter. I assume there were gears inside that changed the speed of the cable so that the mph speedo read in kph. I had one of these on a 1977 Mercury. The interesting thing was that the speedo only went to about 120 so I could bury the needle fairly easy. Pull out to pass and look at the speedo - needle pointed straight down.

Worked fine. Changed the odometer to kilometers as well. Wasn't expensive - I think about $15 in about 1978. Probably possible to make your own or maybe just to change the gear on the end of the speedo drive so that it reads the right value. A kilometer is about .6 of a mile so not a huge change.