weird fucking thought i randomyl had, but i thought of a good strategy to convert km to miles, roughly.

halve the kilometers by 2, then take 1/4 of the resulting number and add it to the resulting number and you have a rough estimate of the amount of miles.

80km / 2 = 40
40 / 4 = 10
40 + 10 is 50, and 80km is roughly 50 miles.

now, going from miles to kilometers, you can also get a rough estimate by doubling your miles, taking 1/5 of that number and subtracting it from the doubled miles to get your kilometer estimate.

50 miles * 2 = 100
100 / 5 = 20
100 - 20 = 80km

this works with other numbers too, often within a margin of error of less than a tenth. give it a shot.