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From: Aidan Kehoe <kehoea@parhasard.net>
Newsgroups: sci.lang,alt.usage.english
Subject: Re: OT: Converting miles/km
Date: Fri, 20 Sep 2024 06:07:19 +0100
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 Ar an naoú lá déag de mí Méan Fómhair, scríobh Christian Weisgerber: 

 > I'm sorry, I don't know where to post this.  I'm crossposting to
 > alt.usage.english, because statute miles as a unit mostly afflict
 > the English-speaking world.
 > 
 > So you want to convert between miles and kilometers.  The conversion
 > factor is... uh...  A 40-year-old calculator book provides a useful
 > tip:  Unless you're designing a space probe, you can use ln(5).
 > 
 > WHAT?
 > 
 > Yes, the natural logrithm of 5 approximates the conversion factor
 > between miles and kilometers; specifically one mile is about ln(5)
 > kilometers.  It's accurate to four digits.
 > 
 > If nothing else, it's faster to type on a calculator.
 > 
 > I think that's hysterical.

The old units were completely neglected when I went to school, which is
unfortunate, it’s routine that we need to convert between vulgar feet and
inches for height to centimetres in daily life, or between vulgar stones and
pounds to kilograms. What I learned from my father (born 1945, went to school
before it was neglected) was that a kilometre is 5/8 of a mile, which helps in
converting the speed limit signs in Northern Ireland to what my speedometer
shows.

I will attempt to bear ln(5) in mind going forward!

-- 
‘As I sat looking up at the Guinness ad, I could never figure out /
How your man stayed up on the surfboard after fourteen pints of stout’
(C. Moore)