It wasn't too long ago that we were announcing that car sales at GM (NYSE:GM) and DaimlerChrysler (NYSE:DCX) were on the rise. At GM, for instance, May truck sales were up 11%. That looked pretty impressive given that gas prices were high enough to induce unprecedented whining by politicians and pundits and even -- prepare to be shocked -- alternative consumer behaviors.

The only May laggard was Ford (NYSE:F), which turned in a dismal 3.1% decrease. Perhaps the venerable automaker was just ahead of its time, because, despite rosy late-June predictions, that month saw an 8% slump, but that was just half the 15% pothole that GM hit. (DaimlerChrysler was just above flat for the month.)

Given results like that, it may be no surprise that Ford and GM have returned to costly giveaways, like the 0% financing, that have been helping to keep them afloat over the past few years. Despite record-breaking incentives in June, foreign competitors like Toyota (NYSE:TM), Honda (NYSE:HMC), and Nissan (NASDAQ:NSANY) turned in robust sales and snatched yet more business from U.S. firms.

If there's one thing domestic carmakers hate, it's losing market share to foreign competitors. If there's one thing shareholders hate -- or should hate -- it's losing margins. Unfortunately, that is exactly what happens when these incentive plans pay out up to $5,000 per SUV in cash and take away the lucrative financing business that have, in the recent past, helped juice earnings.

Frankly, none of the U.S. carmakers, not even GM with its sub-10 P/E ratio, looks like an attractive investment to this Fool. They already sport razor-thin operating margins -- a good 7% lower than Toyota's. If they need to dig deeper in their own pockets just to hang onto their share of the market, it looks like they're not addressing the real problem, whatever that might be.

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Fool contributor and conflicted car hater Seth Jayson saves several thousand dollars a year for his investment accounts by riding a bike instead of driving. He has no position in any company mentioned. View his Fool profile here.