This article is from the RSC/UKSC Cricket FAQ, by Ganesh with numerous contributions by others.
Easy. A mile is 8 furlongs, and a km is 5 furlongs.
So to convert mph to kmph, just multiply mph by 8/5
and vice versa.
Thus 100 mph = 100*8/5 or 160 kmph. 50 mph = 80 kmph 75 mph = 120 kmph.