Clouds loom on the horizon for investors in automotive stocks following a string of lower monthly U.S. auto sales, rising price discounts and incentives, all of which lead to reasonable expectations that automakers will begin slowing or shutting in production to trim vehicle inventories.

So far this year, year-over-year average transaction prices have been down every month, meaning that automakers are spending more to generate less demand. Consumers are marginally less interested in trucks, while sedans are cold. After six straight years of steadily growing unit sales, the market for vehicles in the U.S. -- the world's most profitable -- appears headed down.

Not that the slowdown should be a surprise: Auto sales are historically cyclical and the latest run has been the best ever in terms of length. For investors in automotive equities, the question is how to play the likely spate of bad news as plants shut down, workers apply for unemployment and company profits fall.

Prices for stocks like General Motors (GM) - Get General Motors Company (GM) Report and Ford (F) - Get Ford Motor Company Reportlook cheap if you believe -- as many do -- that the U.S. automakers will emerge from the coming cyclical downturn in strong shape to capitalize. Both sell relatively cheaply on a historical basis and in terms of current price/earnings ratios. But many investors want to be convinced that the managements of both the Detroit-based companiesare capable of cutting costs faster than the decline in revenue and profit.

GM, in particular, hasn't entirely escaped the taint of its 2009 bankruptcy and now is fighting off hedge fund manager David Einhorn and his Greenlight Capital fund, owner of 3.6% of GM. Einhorn has launched a proxy fight; his goal is to split GM into two stocks with the aim of boosting value.

This week Bloomberg reported that Ford CEO Mark Fields is feeling the heat from a board of directors frustrated by the company's low share price. The most frustrated group of Ford shareholders likely is the Ford family, which controls 40% of the votes with about 4% of the company's equity held in a separate class of stock.

TheStreet Recommends

Ford and GM each are investing heavily in advanced mobility initiatives like autonomous vehicles and ride-sharing while maintaining spending on new and redesigned models, a heavy burden that weighs on the companies' short-term results.

Longer term, Ford and GM could find themselves well-positioned in the next few years, assuming a downturn and the advent of advanced-technology vehicles that drive themselves. Sales of conventional cars could return to growth mode just as the advanced technology also is ready for commercialization.

Fiat Chrysler Automobiles (FCAU) - Get Fiat Chrysler Automobiles N.V. Report is a much more heavily leveraged enterprise than either Ford or GM and thus hasn't invested much in autonomous and other advanced mobility enterprises. The lack of investment mightn't be a problem if automotive suppliers and some non-automotive companies in the autonomous field succeed in creating turn-key packages enabling FCA to field driverless cars.

"In the sporting world, two years of falling profit would be considered a losing streak, and I hate losing," Toyota (TM) - Get Toyota Motor Corp. Sponsored ADR Report President Akio Toyoda said this week at a briefing called to forecast a second straight year of down profit for the Japanese automaker. Toyoda's spirited attitude alone might be a reason for long-term investors to give Toyota stock a second look.

Read more auto news on TheStreet:

Doron Levin is the host of "In the Driver Seat," broadcast on SiriusXM Insight 121, Saturday at noon, encore Sunday at 9 a.m.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.