Huh, I don't know how 'common knowledge' this is, but I'm in the habit of keeping a copy of my prize cars at 0 miles in case I need a fresh one for some event online or something. What I did was keep a separate game save in slot 2 and purchased them to that save for safe 0 mileage keeping.
Now, I've just found out that you can actually purchase a car you've just won (or any car in your garage for that matter) onto the same save you're using. Instead of loading your other game and purchasing off of card 1 to get your keeper, you can simply trade from your existing game and load the card 1 garage, buy the car you want and hoo ha, you have two versions of that same car on one save without having to load another game up.
Now, I've just found out that you can actually purchase a car you've just won (or any car in your garage for that matter) onto the same save you're using. Instead of loading your other game and purchasing off of card 1 to get your keeper, you can simply trade from your existing game and load the card 1 garage, buy the car you want and hoo ha, you have two versions of that same car on one save without having to load another game up.