j0057 parent
When driving in the UK, I found myself practicing my 16-table a lot: when seeing a 50 mph max speed sign, I'd multiply 5 by 16 to get 80 km/h.
Given (official*) UK speed limits are always in multiples of 5, using the 8/5 rule would be quicker and work just as well.
* There are occasional places where private property has speed limits with weird factors, but they're so rare as not to worry about. (the one example I remember is when I used to cycle to work and took shortcuts through a sewage plant that had an '11 mph' speed limit, why? who knows.
Hah I've just been driving in France and kept multiplying by 0.6. Much simpler than 16!