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PhilPub
You can use Fibonacci numbers (though obviously they round to integers)
2 m = 3 km
3 m = 5 km
5 m = 8 km
8 m = 13 km
13 m = 21 km
etc.
which gives e.g. 26 miles = 42 km which is close enough for most. Perhaps easier to do addition than multiplication by 1.61 in your head.
Edit: For maths nerds this is because the golden ratio is numerically very close to the mile-to-km conversion factor, and the limit of the ratio of successive terms in the Fibonacci sequence is the golden ratio. Good to 0.5% ignoring the integer rounding.