The bad news keeps coming for Ford (NYSE: F).

Today, the company announced that sales in the U.S. were down 15% in May. Not surprisingly, low-mileage pickup trucks and SUVs led the way. Sales of the F-150, Ford's bread and butter profit machine and its top-selling vehicle for over 30 years, dropped a whopping 31%.

Ford's miserable sales numbers come as Detroit neighbor General Motors (NYSE: GM) announced that it will close four truck plants and maybe even sell its Hummer division.

Both Ford and GM seem to finally be recognizing that their preferred approach to making and selling vehicles -- getting every penny out of their low-tech, low-mileage trucks while avoiding the engineering costs and difficulties of making high-tech, fuel efficient cars -- is finally hitting an immovable wall. Ford's marketing chief, Jim Farley, said that Ford is witnessing "the most dramatic shift in customer segmentation" in decades as consumers move away from trucks. He called May "a watershed month" and claimed that the auto industry is just "catching up with the breathtaking choices that customers are now making."

But it's not the entire auto industry that is just catching up with changing consumer demand. It's really just the manufacturers in Detroit. After all, Toyota just sold its millionth Prius. The silver lining in the dark clouds at Ford can be seen in the fact that Ford's car sales actually rose in May, nearly 3%. The new Ford Focus was particularly strong, selling more than 30,000 units.

Too bad Detroit fought the shift to efficient cars tooth and nail. Other auto companies planned for a more profitable future by building smarter cars. Sadly, the American producers, and especially Ford, are well behind the curve, and are likely to remain so for several years.