Re: OT: Converting miles/km

On Thu, 19 Sep 2024, Christian Weisgerber wrote:

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.

... the English-speaking world with the exception of Ireland, Canada,
Australia, New Zealand, and South Africa and some others.

I think the term “United Kingdom and USA” would have been shorter and
slightly more accurate.

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.

And easier to remember than 1.609344. But when will you need such precision?
For instance, when you drive on German roads outside villages, you must
reduce your speed to 31.0685 mph whereas the rough rule 1 mi = 1.6 km would
have allowed you 31.2500 mph.

I like much more those thumb rules that actually allow you to estimate orders
of magnitude, e.g.:

1 year ≅ π · 10⁷ sec ≅ 31415926.54 sec

or

1 year ≅ √10 · 10⁷ sec ≅ 31622776.60 sec

or, only for those wanting more precision, the arithmetic mean of the two
which has three leading digits correct.

--
Helmut Richter