AutoZone Inc. (AZO) said Wednesday that maintenance products now account for half of U.S. retail sales, as the average age of cars on the road increases.

Executives at the largest U.S. auto parts retailer said there had been a shift towards maintenance products from discretionary and essential purchases in its sales mix.

Bill Rhodes, CEO, said he expected the average age of the cars on U.S. roads was close to 10 years amid the sharp slowdown in new-vehicle sales.

His comments came as the company reported a 9.5% increase in its third-quarter net income.

The Memphis-based company said it earned $173.7 million, or $3.13 per share, an increase from $158.6 million, or $2.49 per share, a year ago.

Shares closed the day down 4.8%, to $155.04, on Wednesday, as investors banked on AutoZone outpacing the results of its competitors, O'Reilly Automotive Inc. and Advance Auto Parts Inc. (AAP).

O'Reilly reported a 36% increase in its fiscal first-quarter net income in April while Advanced had a bigger-than-expected 14% earnings jump.

Rhodes cautioned analysts that the company was planning for conservative growth rate although consumers are holding onto their cars longer.

"The automotive after-market industry association has historically reported overall growth of approximately 4%," Rhodes said on a conference call. "We would encourage everyone to plan growth trends at more reasonable, and more historical, levels."

Auto repair company investors appear to be waiting for a windfall as consumers keep their cars longer amid the recession. In addition, General Motors Corp. (GM) and Chrysler LLC are preparing to close hundreds of dealerships, thereby eliminating service bays in markets.

"As important as our string of 11 straight quarters of double digit earnings per share growth has proven we don't have to achieve these levels of accelerated same-store sales growth to deliver strong growth and earnings per share," Rhodes said.

The company said same-store sales, or sales open at stores at least a year, rose more than 7% during the quarter. The metric is important among retailers because it measures performance at existing, rather than newly opened, stores.

AutoZone's net sales rose 9.3% to $1.66 billion, with U.S. same-store sales up 7.4%. Analysts surveyed by Thomson Reuters were expecting earnings of $2.89 a share on revenue of $1.61 billion.

Rhodes said the company is pushing ahead with its "super hub" concept, where one stores is stocked with more items which it then ships to surrounding stores when needed. "These super hubs have three distinct purposes. Improved market based coverage, improved in stock levels in satellite stores through daily replenishment when necessary, and finally, reduction in excess inventory throughout the network by reducing the need for all stores to carry select products," Rhodes said.

-By Jeff Bennett, Dow Jones Newswires; 248-204-5542; jeff.bennett@dowjones.com