If you want real accuracy, note that the actual mile/km ratio is almost exactly half way between 1.6 and (1+sqrt(5))/2. So do both conversions and take the average.

(You won't get results as accurate as that suggests, of course, because doing the Zeckendorff + Fibonacci-shift thing only gives you an approximation to multiplying by (1+sqrt(5))/2.)

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!