Historically, new car sales has been an excellent predictor of what the economy will do in the future. Since 1970, when new car sales showed a 2% decline year-over-year, a recession has never been much more than a year away. That’s because cars are a product that most Americans need, but are very expensive. People decide to buy a new car only very deliberately, and they tend to buy a new car only when times are good. If a consumer sees his own financial situation deteriorating, he will delay the purchase of a car or buy used.

But recently, car sales as a predictor of the market, and possibly the economy, seems to be not as reliable as it once was. Last year was the best year ever for car sales, according to the Census Bureau. A number of other economic indicators, though, are looking wobbly. And stock market is on the rocks, down just over 6% this year.

The news from the auto industry, though, continues to be good. General Motors on Wednesday reported earnings of $9.7 billion for 2015—an all-time high—on the back of strong SUV and light truck sales. These numbers were certainly buoyed by cheap gas prices, which give consumers less reason to opt for more fuel efficient vehicles that are sold at a lower margin.

And the industry doesn’t see this changing in the near future. Though GM CFO Chuck Stevens says his company is positioned to survive a downturn, “We don’t think it’s going to happen any time soon,” he told analysts on a conference call Wednesday. Other auto industry experts agree. They cite the statistic that the average age of a car on the road today remains near all-time highs and the fact that the latest data on job growth is encouraging. Mark Wakefield, head of the automotive practice at consultants AlixPartners LLP, told the Wall Street Journal last month that consumers were “acting like they just got a raise,” when they hit the showroom.

Despite these rosy forecasts, GM (GM) stock fell close to 2.5%, and is cheaper today than it was when its stock went public again back in 2011. Meanwhile, shares of Ford (F) are down 27% since last year, despite the fact that it has been delivering solid earnings of late too.

So what explains this divergence between auto companies stock prices and record car sales? One explanation is that investors are spooked by shaky data—like declines in overall retail sales and tepid economic growth—and are predicting that the American consumer just can’t keep buying cars at the pace it has been.

That doesn’t mean that the data, as it stands now, are making a strong case for recession. We’re still a long way from seeing year-over-year declines in new car sales that have historically predicted a recession. And though recessions don’t usually take stock markets by surprise, investors can go through periods where their simply overly cautious. As economist Paul Samuelson famously quipped, “the stock market has predicted nine of the last 5 recessions.”

Either investors are seeing something that economists and auto executives are missing, or general uncertainty over the economy is causing them to underestimate Detroit’s strength and the resilience of the U.S. consumer. But if you are hoping to spot a recession before it shows up in the growth data, keep tabs on car sales. Any further declines and both car companies and the broader economy could be in for a rough ride.