I find this way of converting super useful. I've used it many times since moving to the UK. For example, the 30 mph speed limit is roughly 50 km/h.
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A Mathematician's Way* of Converting Miles to Kilometers
• 1 mi ≈ 1.609 km
• The ratio of consecutive Fibonacci numbers Fₙ₊₁/Fₙ tends to the golden ratio φ ≈ 1.618 as n increases
So Fₙ mi ≈ Fₙ₊₁ km
*This is how I do it 
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Wow, that's actually an awesome reason to memorize the Fibonacci sequence!
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1 mi ~= 1km 1 mi ~= 2 km . . .
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Yeah, but you don't really need any tricks to convert powers of 10. 1000 mph ≈ 1,600 km/hr.
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How I've always done it. It used to work for dollars to pounds too.
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Soon to be an even easier conversion of 1 to 1
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It’s so straightforward! How did I not think of that?
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Because it's a coincidence and has no mathematical basis?
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