Consumer spending accounts for about 70 percent of the U.S. gross domestic product (GDP). Accordingly, there's a lot of interest in consumer behavior, so economists have developed many ways to evaluate the health of consumer spending.

Among them include rigorous surveys that track consumer sentiment and confidence, interest rate tracking particularly around government bonds, indexes that track how prices change over time, and employment data such as job openings and unemployment claims.

The challenge in recent years is that the "tea leaf" reading of economic data has been all over the place. Forecasts have become less reliable—something is missing.

One area that might offer clues is the automotive aftermarket. It's an underrepresented area of the economy that has an enormous impact on consumers. The automotive aftermarket may well be the biggest market you have never heard of.

Pre-Owned Vehicle Sales Easily Outpace New Vehicle Sales

America is highly dependent on our vehicles. Around 93 percent of Americans who leave the house for work commute in a vehicle, including cars and light trucks, according to U.S. Census data. Overall, Americans own a whopping 291 million vehicles. Those vehicles are how we get to work, bring our kids to their extracurriculars, run errands, attend church, and conduct many of the other daily routines in our lives.

Most of those cars are........

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