Detection depends on how smart you are (archive)

I have lost total faith in expecting a used car to have legitimate miles on the odometer. They(the manufacturer of this device) even go as far as saying that it cant be detected when the odometer has been wound back with their device!!

As I have been into programming interface software for engine diagnostic units for some time, I think I can say that it is possible to perform such changes without traces.
I can only speak for VW/Audi/Seat/Skoda as far as my experience reaches, but anything that can be changed can be changed.
The protocols themselves are very complicated, the baud rates uncommon (down to 5 - yes, five - baud) and the command words are of course not publicly available. And I'm not even mentionning checksums here.
It is impossible (so I dare to say) to achieve all this information by trial-and-error. So the guys who are building such devices must have (had) some insight into documentation. With that knowledge you're able to do everything you like - from reading error messages and changing idle speed to hilarious values to finally changing the mileage. Been there, done that.

...Mike

P.S.: If done improperly, there will be errors and the engine management might refuse to work.

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