The idea is that a hypothetical Lamborghini-index would give you information that is essentially a tiny subset of what you'd already know from GDP+GINI. So you

The idea is that a hypothetical Lamborghini-index would give you information that is essentially a tiny subset of what you'd already know from GDP+GINI. So you can compare GDP+GINI to GDP+GINI+luxury cars and conclude there is no advantage of having the extra information. It basically tells you "there are at least n people in this country who can afford a Lamborghini, furthermore if you assume that r percent of people who can afford a Lamborghini actually buy one, then this country has approx m=r*n such people". But basic economic data already lets you estimate m fairly well, so what's the need for knowing n? Perhaps if r varies a lot between countries knowing the actual number of cars gives you some extra information about how much that country in particular values cars as symbols of status. But, would that be useful? Either way, this is very tangential to the point I was trying to make, which is that education and cars are not equal measures of the development of a society and that an educated population is more that some sort of social status scorecard :)