Someone (a jogger/runner?) on reddit a few days ago made an interesting observation, maybe this was known. The Fibonacci sequence can be used as a rough guide to convert km to miles and vice versa.
Nothing mysterious there; in the Fibonacci sequence, the ratio of the number and the one before it converges to the golden ratio which is almost the same as the conversion rate between km and miles.
I think it is easier to multiply by 0.6. Or do the simpler approximation: n/2 + n/10 + ( n/100 )*2
You can drop the last term if you don't need an accurate result.
Divisions and multiplications by 2, 10 and 100 are really easy.
850km to miles:
425 + 85 + 17 = 527 miles , the correct result is 528.166 miles
258km to miles:
129 + 26 + 5 = 160 miles , correct result is 160.314 miles
0, 1, 1, 2, 3, 5, 8, 13, 21, 34
The first four don't work very well but:
3km is about 2 miles
5km is about 3 miles
8km is about 5 miles
13km is about 8 miles
21km is about 13 miles
34km is about 21 miles
and so on