i'm not certain, but i've always assumed it was based on tire rotations. you know the circumference of a tire to a fairly good estimate.

let's say the tire has a 3 foot circumference (that won't be the actual number, but it's good enough for an example).

3 feet * number of times the tire has gone through a full rotation * 1 mile/5280 feet = miles the car has been driven.

you can do that digitally or you can set up a mechanical gear train to do the conversion for you.

not certain that's how it's actually done, but that's how i'd do it.

Paul

When in doubt, think about penguins. It probably won't help, but at least it'll be fun.