Oh, well. It was fun while it lasted, no matter how brief it may have been. After last month's surprise gain in U.S. sales, General Motors (NYSE:GM) is once again back to its old ways. For March, the automaker reported a 4.2% decrease in U.S. sales. To make matters worse, there was even an additional selling day in this year's period. Adjusting for the extra day, GM's sales were off 7.7% from last year. So the streak is over ... at one. Not exactly DiMaggio-esque.

What could have brought GM's impressive run to a screeching halt? It took a couple of factors. First, total truck sales dropped 7.7% for the month year over year, although the company did say sales of its Silverado and Sierra pickups exceeded its expectations in the first quarter.

The second cause of reduced sales is one that I'm much less concerned about. GM continued its efforts to reduce fleet sales, which provide limited profits. Although this move obviously hurts its sales results in the short term, I still think cutting back on fleet sales is the right move for the company.

In a reverse of last month, car sales rose compared with last year, inching up 2.4%. However, that wasn't nearly enough to make up for the combined drop in truck and fleet sales. As was the case last month, Saturn shone the brightest, increasing sales by 22.5%. Unfortunately, its Buick lineup more than offset that gain with a 32% drop.

Despite the falling sales, GM maintained a strong lead over rival Ford (NYSE:F), which saw its sales drop by 9% over the same period. GM is also well ahead of Toyota (NYSE:TM), although the Japanese competitor continues to rapidly gain ground in the market; it increased U.S. sales by 11.7% year over year.

These trends of up and down sales and market-share adjustments are likely to continue for the foreseeable future. The good news is that GM has seen great success from its newly redesigned models. The bad news is Toyota continues its tear with no signs of slowing down just yet. GM will have to continue to get accustomed to sharing more of its sales and must not lose focus on turning out designs that consumers demand. If it can continue to do that, hold down costs, and avoid spending too much time looking in the rearview mirror, GM should survive and should be able to start a legitimate streak of good results.

To learn more about what GM and its competition have been up to, read:

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Fool contributor Mike Cianciolo welcomes feedback and doesn't own any of the companies in this article. The Fool has a disclosure policy.