Auto sales are perking along, growing faster than the rest of the economy, and brightening the financial prospects of the three big Detroit automakers.

Look behind the dollars, however, and the news isn't so encouraging. After seemingly stabilizing their 2011 home-market positions, General Motors and Ford are losing market share again. The losses highlight some familiar facts: Outside of some traditional domestic strengths like pickup trucks and pony cars, Detroit is having a hard time competing head-to-head against ever-strengthening foreign competitors.

Whether the slump is temporary or the continuation of a five-decade slide isn't clear yet. Some see the U.S. market increasingly coming to mirror Europe's of the 1990s, when six automakers each controlled a similar share of the business. Regardless, it means that Detroit has to shore up its core model lines while it fights off competitors in battleground segments like mid-size sedans and small crossovers. Some sources of worry:



NEXT: 1. It's worse than expected

