The US auto industry is approaching a critical turning point

On Tuesday, the major automakers will report their US sales for July.

Expectations are for a strong month. No analyst regularly surveyed by Bloomberg anticipates a yearly sales pace below 17 million, even though June's pace was only 16.66 million.

That followed a 17-million-plus pace for 2016 through May, which itself followed a record 2016, with 17.5 million total new car and truck sales.

But there are reasons to start getting nervous about the sales boom, which has been driven by cheap gas, easy credit, a solid employment picture, and an average vehicle age on the US roads of 11 years.

Last week, Ford reported second-quarter earnings, and while the carmaker reported a record North American profit for both the quarter and the first half of the year, the quarterly number came in a little light, sending Ford shares into a tailspin, down as much as 10% at one juncture (they still haven't bounced back).

General Motors and Fiat Chrysler Automobiles both beat on their earnings, which came in prior for Ford's. But all three Detroit automakers have been riding the same SUV wave. With talk of the US market flattening, the worry is that automakers will hike incentive spending to hold or gain market share, effectively throwing away profits that could be reinvested or used to buyback shares or raise dividends.

If July sales come in at around a 17-million pace, it will look like the market is plateauing. If the numbers comes in at 17.5-18 million, there may be some growth left in 2016 and the concerns will fade.

But if we slip below 17 million again, all the fretting about incentives will immediately intensify and the stock of GM, Ford, and FCA could all be punished.

There isn't much question about what vehicles will be selling best. They're pickups and SUVs. In a flat sales market, that's a plus, because carmakers can make more money with that type of product mix.