Comparable-store sales growth is the mantra of all retailers. But when the top line hits a speed bump, it gets tougher to bring home the bottom line.

Reviewing the last four quarters, O'Reilly Automotive (Nasdaq: ORLY) has proved this in spades. The company really knows how to bring home the bacon when comps are strong. But when top-line growth decelerates, earnings growth seems to fall in tandem.

% Change

Q1

Q2

Q3

Q4

Comp Sales

6.8%

2.0%

4.3%

2.1%

EPS*

18.3%

4.4%

9.6%

0.0%

Source: O'Reilly Automotive earnings releases. *Earnings per share.

Earnings growth skidded to a halt despite the fourth quarter's 2.1% increase in same-store sales and 8.2% rise in sales. The culprit?

My first guess would have been that O'Reilly was the victim of one-time costs relating to its on-again, off-again attempts to acquire CSK Auto (NYSE: CAO). However, the main contributor to the earnings stall actually was higher operating expenses, which rose 110 basis points, mainly thanks to higher store payroll and a loss of leverage resulting from a rapid sales slowdown.

O'Reilly has been coasting for some time now, as customers are scaling back discretionary spending. While AutoZone (NYSE: AZO) showed last quarter that it can still burn rubber, rivals are showing signs of industry weakness. Advance Auto Parts (NYSE: AAP) just reported negative comps for its most recent quarter, while guiding analysts toward flat results in 2008. Meanwhile, Pep Boys (NYSE: PBY) is in the midst of restructuring its business.

To me, O'Reilly has the best long-term growth prospects in the sector, it just needs to tighten its belt a bit on the expense side. If the company can remain disciplined in its spending habits, O'Reilly should be positioned to thrive when the economy finally ignites its growth engine again.

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