DETROIT ( TheStreet) -- First the good news: GM's ( GM) March sales rose 6.4%, ahead of the industry's 3.4% gain. GM had its best March in five years. Its first-quarter market share rose to 18% from 17.5% in the first quarter a year earlier. Cadillac sales rose 50%. Buick sales rose 37%. And GM China sales hit a first-quarter record of 816,373. The future looks good because GM will begin selling its new K2XX Silverado and Sierra pickup trucks toward the end of the current quarter, just as the pickup truck market seems to be gaining steam as both an indicator and beneficiary of the resurgence in housing construction. The bad news? Start with the share price. GM rose 3% in March, but it is down 5% this year, while the S&P 500 index is up 9% and Ford ( F) shares are down 4%. Ironically, automakers' shares are down even though industry experts say the stock market's rise is among the factors contributing to strong U.S. auto sales. Also, in terms of March pickup truck sales, GM trailed every other manufacturer. Additionally, while GM's U.S. March sales grew 6.4%, Bloomberg's consensus estimate was 12%. GM's first-quarter market share was up, but as Jefferies analyst Peter Nesvold wrote in a note, March market share was trending down to 17%, below its 18.2% market share on a trailing three-month basis. Perhaps the share trends are a sign of weakness, or perhaps they are a sign that Wall Street follows automotive sales a little bit too closely. What is clear is that potential GM and Ford investors are so far focused more on Europe's bleak auto economy than on the U.S. auto economy's promise. "The biggest factor weighing on these companies is Europe," said S&P Capital Markets analyst Efraim Levy, in a recent interview. "As you get more clarity on a turnaround in Europe, on if that will happen and when, it will relieve pressure on the stocks," Levy said. "In the U.S., both companies are doing well and are gaining market share." Still, GM faces questions about its March performance in the U.S. pickup truck market, which, after all is said and done, remains at the heart of the Detroit Three's business model.