While many fast-food chains are struggling to increase their customer counts, Sonic (SONC) can't feed its customers fast enough. Sonic's fiscal fourth-quarter results show restaurants with growth figures that most of its competitors could only dream about.

Results
During the fourth quarter ending Aug. 31, Sonic grew adjusted earnings per share 20% from $0.25 to $0.30. The key driver was a 5.9% increase in same-store sales. Management is focused on even more growth in 2014, which it expects to achieve by way of more media, more products, and more promotion. In other words, doing what it's been doing successfully, but stepping it all up.

Additionally, Sonic has slated "new technology investments to enhance the guest experience" in 2014. These investments are expected to increase sales and margins at the same time. Total revenue grew from $151 million to $159 million. Almost all of that increase was from same-store sales. Sonic is focused on its individual restaurants and not so much on growth. It has around 3,500 restaurants, but only added 27 new ones over the last year, which is an increase of less than 1%. All but two of them were added by franchisees.

Sonic continued with positive forward-looking comments in its conference call. CEO Clifford Hudson stated:

"...we look to continue growth of earnings per share."

He then listed the items that led to the 20% EPS rise and what will allow it to continue to rise:

"...same-store sales, operating leverage of the company, new store development" and "ascended royalty rate."

Additional plans to accomplish this going forward include "more effective media strategy" and to "continue innovation of our product pipeline." So far so good, as evidenced by its results.

Competitors
Sonic reports strong same-store sales growth at a time when other big chains such as McDonald's (MCD -0.42%) and Yum! Brands (YUM -0.18%) are looking at weak growth at best.

McDonald's last posted a paltry gain of 0.7% in the US. In part, it blamed "broad-based challenges of the current environment" and said that "the Company expects the dynamics of the current environment to persist" while forecasting flat sales for the month of October. Meanwhile, in neither its earnings release nor its conference call did Sonic about the economy being a challenge. Sonic doesn't seem to see what McDonald's sees.

In its conference call, McDonald's revealed a bit more of a bleak future. It admitted that fourth-quarter sales were an easy comparable to beat, yet still wouldn't. Its CEO Don Thompson stated:

"...macroeconomic and competitive conditions are not expected to change dramatically. As a result, we expect October sales to be relatively flat."

McDonald's did a lot of outward blaming throughout the call. Phrases such as "tough macroeconomic conditions," "the economic uncertainty, cautious consumer sentiment, increased taxation, and lower disposable," and "a slowing of the economy" kept popping up. McDonald's used the word "challenge" nine times in the call. The total number of times Sonic used the word "economy" or "challenge" to describe the environment was zero.

Yum! Brands had similar growth problems to McDonald's. Internationally, same-store sales were only up 1% while in the US they were flat. On the one hand, CEO David Novak stated:

"We never have blamed the macros for anything. We don't talk about macros, or weather, or anything like that. We just don't. We just don't believe in it."

On the other hand, it seems like Yum! Brands slipped up and revealed concern about the "macro" problems. Phrases popped up such as "...the country macros and our recent results haven't been as strong as we'd like," "despite current challenges," and "2013 is a very challenging year for Yum! Brands." Clearly Sonic is succeeding at growth at a time when others are struggling to achieve it.

Final foolish thoughts
Sonic is growing across all metrics as if the economy was in full bloom. Watch the economic indicators, particularly consumer spending; if and when it strengthens, Sonic's already fantastic growth rate could accelerate. Also continue to monitor Sonic's same-store sales and management comments about its various company-specific growth efforts to make sure they are producing as intended. Even in the current challenging environment, Sonic represents a potentially lucrative investment opportunity.