The United States is 4% of the world population. It consumes 25% of the oil in the world. Most it goes to transportation. So by monitoring the status of transportation one can get an idea of how well the economy is doing.
This is just the total vehicle being sold. Let us look at the loans associated with these purchases.
And to finish the picture we have to consider also the total registrations. This is going to tell us if we are adding cars or driving more cars with more expensive price tags.