[cryptonector]

The conversion factor is very close to the golden number, so if you just remember the Fibonacci sequence up to around 89 you'll be able to do mental conversions easily enough.

E.g., what's 65mph in kmh? Well, the nearest Fibonacci number to 65 is 55, so 65 == 55 + 10, and the next Fibonacci number to 55 is 89, so 65mph will be 89kmh + ??, now we turn to 10mph and notice it's 8 + 2, so it's really 13kmh + 3kmh, which brings us to 65mph ~= 89kmh + 13kmh + 3kmh = 105kmh. My calculator says the actual conversion of 65mph to kmh is 104.60736, so... that's really close!

This is a very fun little mental exercise to do. Obviously it works in both directions.

Let's apply it to 340mi. The bracketing Fb numbers are 233 and 377. 377 - 340 == 33. Next after 377 is 610, so 377mi ~= 610km. Next after 33 is 55, so 340mi = 377mi - 33mi ~= 610km - 55km = 555km. My calculator says it's 547.17696km. Close!!