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Comment by gambiting | Hacker News Reader

I've got caught by this, took some explaining to straighten out. I always have my on board computer set to display distance in kilometres, but I live in UK - and once, a technician logged my mileage using the value in kilometres, but for the next service they switched to miles - and I got some questions from Mercedes how come my mileage went down between visits to the garage.

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For curiosity's sake, if you're interested you can see how often mileage is reported incorrectly by looking at the MOT data set published by VOSA: https://data.gov.uk/dataset/anonymised_mot_test.

the scary thing is that they didn't figure it out themselves

but if they did just make the assumption that any odometer reading that was off by 0.62 must have been a km->mi error, couldn't someone attempting to rollback an odometer just always make sure it was rolled back to 0.62 x the number of units driven?

The issue is obviously that the two readings were not taken at the same time, and the technician does not have a field for the unit - just for the value. So let's say he typed in 20,000(in kilometres, so about ~12k miles), then I drove another 5000km, so the next reading taken was 15,000(in miles). So all they could see on their system was my odometer going down by 5000 "units" - it's impossible to work out from this that the mileage was taken in different units each time.

Right - that was my point to the parent who said "it's scary they [Mercedes] didn't figure that out" (that there was a unit change, and thus proceeded down the road of "must have been a rollback"). I was trying to get across that simply making the assumption that mileage going down implies a kilometers-to-miles conversion in reading is ripe for abuse, and so Mercedes would have to assume a rollback first and investigate.

Point taken, but they first could have investigated a unit mixup before assuming fraud. Or they could always track mileage with explicit units, not assuming a default.

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