The 12% approximates to the average gross margins of the dealers I prepare accounts for, though this will of course include margins for servicing etc. Margins are generally around this level mainly due to manufacturer discounts in addition to dealership bonuses etc, although I will admit that the margins on different models of car vary greatly with some substantially higher and some lower.
I would point out that net margins are tight and a lot of dealerships would barely break-even without the manufacturer incentives.
Hope this helps.