Another note here...
Something I noticed is that... When I race online with my friends we usually just race Nascar at Daytona. So, I usually go the whole night in one car...
The miles do not add to that car until I get into another car and the game does the autosave. And.. somtimes, after an oil change, the cars mileage will decrease.
This is referring to the miles on the car itself, I haven't seen how my overall miles are affected.
