the car would only tip when the board gets to vertical if the car's centre of gravity was at 0mm off the ground, obviously it is higher than this, so the car will tip when the centre of gravity is directly above the outside edge of the wheels that are at the lower end of the slope.
Imagine standing a book up on a board and doing the same, it would obviously tip way before the board got to vertical, the average car would have a much lower C of G / wider stance than the average book, hence the board would be closer to vertical (but not actually vertical) for the car than the book.
I saw it in a C of G comparison between Tamiya Clodbuster and a Kyosho USA1 monster trucks, the magazine put both on a board, lifted it and saw which one tipped first.
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