OKLAHOMA CITY--(BUSINESS WIRE)--Sonic Corp. (NASDAQ:SONC), the nation's largest chain of drive-in restaurants, today announced system-wide same-store sales for its fiscal 2015 of 7.3%. Same-store sales growth reflected an increase of approximately 6.9% at company drive-ins and 7.3% at franchise drive-ins for the year ended August 31, 2015. The Company also announced that system-wide same-store sales for its fourth fiscal quarter increased approximately 4.9%. Same-store sales growth reflected an