What's wrong with...
Miles to Km: Miles x 8/5
Km to miles: Km x 5/8
Accurate enough, especially with lower values.
Nothing wrong with it but, being a bear of very little brain I find divisions and multiplications by 8 hard to do in my head.
Yeah, there are shortcuts but I don't find it all that easy. So I rely on my method which only requires divisions by 2 and 10 which are easy.
And if I'm not doing it in my head then I just multiply or divide by 0.6214
Plus, of course there are various "signposts" that I have by rote from things like riding audaxes (eg 200km = a wee bit less than 125 miles), and speed limits in France (eg 110km/h is a tad under 70)
