If the relationship holds, it spells bad new for the already weakening second half recovery meme.

"We've never seen a plunge this steep that didn't foretell a recession," says Bloomberg columnist David Ader.

For most people, the economy's ups and downs are best measured by famous indicators like monthly job reports and quarterly releases of gross domestic product. But students of the arcane took special notice earlier this month when the Bureau of Economic Analysis released some disturbing data that didn't make anybody's front page. In August, domestic heavy-truck sales fell 29 percent from the same period of 2015, the weakest month in well over three years.

Any drop that dramatic could always be an anomaly, but heavy-truck sales have been slipping for two years. Broad weakness in this category has historically been a reliable hint that a recession is on its way.