Tamarlane has a great question no one has answered yet, what is the Monopoly race car based on? An early Indy car? A streamlined lakes racer?

From http://karakullake.blogspot.com/2010/05/monopoly-game-car-piece.html for info on the games pieces, according to the following link the design changed several times over the years anyway:
http://www.monopoly-history.com/tokens.php