Miles to kilometres

Adding sixty per cent is the shortcut, and it is unusually well behaved. Sixty miles plus sixty per cent is 96 against a true 96.56; two hundred gives 320 against 321.87. The error is a steady 0.58 per cent at every scale, so it never surprises you by getting worse on a long number.

A speedometer is not allowed to flatter you downwards. Under UNECE Regulation 39, which the European Union and the United Kingdom both apply, the indicated speed may never be lower than the true speed and may exceed it by no more than ten per cent plus 4 km/h. At an indicated 70 mph the car may genuinely be doing anything from about 61 mph upwards. That asymmetry is deliberate, and it is why a satnav reading, which comes from GPS rather than from a wheel sensor, usually sits a little below the dial.

The mile itself is a Roman unit that survived a translation. Mille passus was a thousand double paces of a marching legion, roughly 1,479 metres. The English version drifted and was pinned down by a statute of 1593 at eight furlongs, or 1,760 yards, a number chosen to keep the furlong intact because the furlong was tied to the length of a ploughed field. The metric value arrived centuries later, once the yard had been defined against the metre.