I always just use either 6/10 or 2/3, depending on which one is easiest to do in my head.
Yes, this means both 90 and 100 km/h both covert to 60mph. Close enough!
Yes, this means both 90 and 100 km/h both covert to 60mph. Close enough!
So the km:mi ratio is somewhere between 60:100 and 66:100. I choose whichever one makes sense depending on the number in front of me.