AutoZone ( AZO) is shifting out of overdrive.

With its old growth engine sputtering, the nation's largest auto-parts retailer is looking for a new way to soup up its performance. The company, which zoomed to prominence by catering to retail customers, is now steering its focus toward the commercial market.

AutoZone hopes to grow at an "accelerating pace" by rolling past smaller players and snatching up more of the $47 billion commercial market that, so far, it has barely tapped. Right now, AutoZone still relies on retail customers for roughly 90% of its business. But the company has become increasingly dependent on commercial sales -- which rocketed 30% in the latest quarter -- to fuel much of its growth.

"There's tremendous opportunity to gain market share" in the commercial sector, AutoZone CEO Steve Odland told investors in a conference call on Wednesday. "We're really confident in our model."

But some investors clearly prefer the model in the rearview mirror. They took one look at AutoZone's same-store sales for the third quarter -- flat at the retail level and up just 2.8% overall -- and sped away. The stock, trading near historic highs ahead of Tuesday night's earnings release, tumbled 3.3% to $84.50 Wednesday. Even a fresh buy recommendation from Legg Mason, which declared the quarter a strong one overall, couldn't stop the slide.

AutoZone CFO Mike Archbold insisted Wednesday that the company "hit it out of the park this quarter" and declined to speculate on the market's reaction.

"People can choose to buy or sell," he said. "But we did beat expectations. ... I think it was, overall, just a great quarter."

Knocks and Pings

But the market dwelled on AutoZone's weak points. Although the company managed to grow third-quarter earnings by 23%, it topped Wall Street expectations only with help from an unexpected 3-cent gain. Counting the gain, the company reported third-quarter profits of $1.30 a share instead of the $1.27 most analysts were anticipating.

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