Everyone is talking about how fast-casual chains like Chipotle and Five Guys are stealing McDonald's customers.

But there's another huge competitor no one is talking about.

Gas station/convenience store businesses like Sheetz are taking market share from the fast food giant, Michelle Greenwald, a marketing professor at Columbia Business School, told Business Insider.

"Gas stations like Sheetz have gone through the effort to make sure people like the food," Greenwald said. "They're also very convenient for families and have a one-stop shop advantage."

And unlike Chipotle, which is much more expensive than McDonald's, gas stations are closer to the Dollar Menu price point.

Sheetz offers a variety of food, including pizza, wings, sub sandwiches, pretzel melts, hot dogs, salad wraps, and more.

President and CEO Joe Sheetz has been open about the emphasis the company places on food.

"When we’re new to the area, gasoline gets their attention," he told PennLive.com. "Once you get their loyalty, we get them to eat. We are really a restaurant advertising a bunch of other stuff."

The chain has about 500 locations concentrated in Pennsylvania, Virginia, Maryland, West Virginia, and Ohio and is expanding fast.

Sheetz also has a full-service espresso and smoothie bar for those who might frequent the McCafe. The bar is staffed by trained baristas.

"Their Made-To-Order Espresso bar is actually comparable to Starbucks, but with half the price," writes one Yelp reviewer.

Greenwald also notes that the customer service at these chains tends to be better than McDonald's.

"Workers at Sheetz are encouraged to smile and greet people as they come in," Greenwald said. "This makes people comfortable and keeps them coming back."

The convenience is another huge factor for customers.

Sheetz is open 24 hours a day, 365 days a year.

"Cheapest gas prices... combined with 24-hour service and proven to satisfy your [Made-To-Order] fixes every time. Really, why go anywhere else?" Yelp reviewer wrote.

US customers have been fleeing McDonald's as the company faces a growing perception problem and slow customer service. CEO Don Thompson will leave the brand in March.