So cool: if your miles are the lower value in a Fibonacci sequence, the kilometers are approximated by the next value.
FTNItK: the Fibonacci is what you get when you start with 1 and 1, and the next value in the series is always the sum of the last two. So, 1 and 1 are followed by 2, then 3, 5, 8, 13, 21, 34 and so on.
So, 21 miles is about 34 kilometers.
How accurate is this approximation? Within one percent for small numbers, and within a half percent once you get past that 21.
Do note though, that this is just a coincidence.