When you register, I think the web site provides a question that asks you what your normal annual mileage is. At least that is what I remembered when I registered. The site is probably calculating your current mileage from that. I'll post this to the GM NG...someone there may know more about this. Although you do raise a interesting question.
I registered my Buick on the GM web site when I purchased it new about
>2-1/2 years ago. I checked the web site today for the very first time
>since registration.
>
> To my surprise they pegged the mileage on the car within 100 miles. >
> I have the OnStar package but do not subscribe. Are they monitoring the
> mileage via this link or do they have a "very good" algorithm that can
> figure it out. I haven't had the car into the dealer in over six months. >
> Just curious.....
>