Fast-food chains are gaining muscle again

McDonald's (MCD), Burger King and Wendy's (WEN), which have struggled in recent years, are now dishing up some appetizing operating results.

Same-store sales, a key metric of sales at locations open a year or more, have been on an upswing for the big chains recently. For instance, that figure has risen 5.7 percent at McDonald's over the past 13 months, by 3.9 percent at Burger King over the 2015's last quarter and by 4.8 percent for Wendy's in the same period.

According to Darren Tristano, president of restaurant consulting firm Technomic, consumers are spending more at the chains, thanks to lower gas prices and an improving job market. The companies are also selling their food more aggressively to budget-conscious diners, a key demographic for the industry.

Burger King parent Restaurant Brands International (QSR) benefited from remodeling the burger restaurants and the expansion of the Tim Horton's donut shop chain, which it also now owns. During yesterday's earnings conference call, the company said U.S. franchisee profitability rose by more than 30 percent over last year, which CEO Daniel Schwartz called a "tremendous accomplishment." Franchisees, who are independent business operators, own many fast-food restaurants."With so much advertising shifted toward value play, $4 for 4, $2 for 2, etc ... low prices are driving consumers toward convenience, value and comfort food," he said, adding that renovations at the chains have also paid off. "Locations are becoming more appealing to consumers, who have viewed these restaurants as old and outdated."

Restaurant Brands has high hopes for an American classic: Grilled hot dogs, which Burger King is rolling out at more than 7,000 U.S. locations later this month. It may be chain's largest new product launch since the 1970s.

"I personally visited the test market to confirm that the Grilled Dogs could be an operationally simple but pretty impactful product," Schwartz said during the conference call. "And we're all excited about it."

Restaurant Brands was created in 2014 after the $11 billion acquisition of Tim Horton's by Burger King Worldwide, which is controlled by Brazil's 3G Capital. The transaction, called an inversion, lowered the company's tax bill because it relocated to Canada, and it remains controversial.

Restaurant Brands on Tuesday reported better-than-expected profit, excluding one-time items, of 35 cents per share on revenue of $1.06 billion. Same-store sales rose by 6.3 percent at Tim Horton's.

Wall Street, though, remains skeptical. Shares of Restaurant Brands have slumped more than 18 percent over the past year, underperforming McDonald's, which gained more than 23 percent during that same time amid investors' enthusiasm of a potential turnaround at the Home of the Golden Arches.

Morningstar analyst R.J. Hottovy, however, argued in a recent note that investors were overlooking Restaurant Brands' potential for growth.

"While McDonald's turnaround may have generated the most quick-service-restaurant headlines the past several months ... Restaurant Brands International continues to fly under the radar with effective menu strategies, new franchise partnerships across the globe and exceptional cost discipline," he wrote.

Earlier this year, McDonald's reported its strongest quarterly earnings in nearly four years as consumers responded to the chain's decision to offer breakfast all day. Wendy's results beat Wall Street's analysts' expectations, and the chain forecast better-than-expected sales at existing locations in 2016.

Other fast food chains are also doing well.

Yum Brands (YUM), the parent of Taco Bell and KFC, recently reported better-than-expected quarterly profit, though revenue growth was hurt by the sluggish performance at Pizza Hut.

Popeyes Louisiana Kitchen (PLKI) announced in January that it expected 2015 per-share earnings to be better than it had previously forecast. It plans to release results on Feb. 23.

If the industry keeps this momentum going, investors may soon start ordering more fast-food shares.