Apple fans are already hyperventilating with excitement about how much money Apple is going to make selling cars. So let's put some numbers around that. What if Apple immediately builds a car business the size of, say, Porsche's? How much more money will Apple make? Well, Apple made about $53 billion of operating profit in the fiscal year ending in September 2014, a spectacular 28% operating profit margin on revenue of $182 billion. Porsche, meanwhile, made about $2.6 billion of operating profit, an impressive (for a carmaker) 20% operating profit margin on revenue of $14.3 billion. So if Apple sprouts...