If you want to quickly convert between miles and kilometers,
you can use the Fibonacci sequence to make a conversion with a
good degree of accuracy. The Fibonacci sequence, which is the
basis for the golden ratio (1.618), begins like this: 0, 1, 1,
2, 3, 5, 8, 13, 21, 34, 55, 89, 144
Each number in the sequence after the first two is the sum of
the previous two numbers. Look at any number in the sequence as
a number of miles and the next number in the sequence is
approximately that same distance in kilometers. For example, 55
miles equals about 89 kilometers. Conversely, 34 kilometers
equals about 21 miles.
The conversion of miles to kilometers is 1.609. The small
difference between that and 1.618 will cause higher numbers to
not equal, but the numbers would need to be quite high to make a
difference. When in your car, check your speedometer for a quick
comparison.