i have the michelin 1:200,000 road atlas of france.

being of a boy-scout turn of mind, i decided to measure the grid to get a grip on the overall scale of the atlas; grid lines are spaced at 65mm (+/- 0.5mm measurement error) which yields the bizzare spacing of 13km on the ground.

13 kilometers. why 13 kilometers?

well, for one thing that is almost exactly the same distance as 8 miles, the latter a far more divisible number, especially to a geek.

so: can anyone explain why michelin maps are labelled with 8mi / 13km grid squares? some pre-metric hangover, perhaps?