Inspire Brands, the private equity backed firm that owns Arby's, Buffalo Wild Wings, and Rusty Taco, has added another fast-food giant to its stable: Sonic, the retro-inspired hamburger drive-in chain. Faced with competition from McDonald's, Burger King, and the like, Sonic's sales have been suffering. Inspire bought it for $2.3 billion, almost 20% more than what its shares were trading at Monday, per CNNMoney. The firm says it plans to run Sonic, which has more than 3,600 locations, as an independent brand and that nothing major will change under the new ownership. The deal was announced Tuesday, and Sonic's shares rose more than 18%, per CNBC. Sonic is the nation's largest drive-in chain, Business Insider reports, and is known for quirky menu items including the pickle slushie. (More Arby's stories.)