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From: wollman@hergotha.csail.mit.edu (Garrett Wollman)
Newsgroups: sci.lang,alt.usage.english
Subject: Re: OT: Converting miles/km
Date: Fri, 20 Sep 2024 17:43:01 -0000 (UTC)
Organization: MIT Computer Science & Artificial Intelligence Lab
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In article <874j6agbg8.fsf@parhasard.net>,
Aidan Kehoe  <kehoea@parhasard.net> wrote:
>
> Ar an naoú lá déag de mí Méan Fómhair, scríobh Christian Weisgerber: 

> > 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.

>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.

That's the approximation that's much easier for mental arithmetic, and
similarly, 8/5 when converting the other way.  Otherwise, 1.609344 is
the exact conversion, and 1.609000 is good enough for almost every use
(unless you're doing weird stuff like survey miles, which are based on
the old foot of 1200/3937 meter,[1] rather than the "international"
foot of 0.3048 m).

However, an approximation that has actually proved useful to me is
that 1 m/s ≅ sqrt(5) mi/h.  As everyone knows, sqrt(5) ≅ 2.236068, or
2.2 if you only need two significant figures, which is much easier
to remember than 3600/1609.344 ≅ 2.236936.

-GAWollman

[1] One will note that this conversion defines a meter to be exactly
39.37 inches, although that's never how it's actually phrased.
Sometimes that's a more convenient conversion than the modern one even
if it's no longer exact.
-- 
Garrett A. Wollman    | "Act to avoid constraining the future; if you can,
wollman@bimajority.org| act to remove constraint from the future.  This is
Opinions not shared by| a thing you can do, are able to do, to do together."
my employers.         | - Graydon Saunders, _A Succession of Bad Days_ (2015)