@Lurk Rambis what model mustang you driving? Premium gas isn't a ploy lol. New engines are just so advanced that they will adjust timing(plus all the knock sensors help) if you put a lower octane gas in your car. If your car requires premium fuel it's best to use premium. By putting regular in your car you're 100% retarding the performance of your car.
I put 87 in my SRT one day by accident cause I was use to filling up my daily driver Mazda 4 banger. Hadn't drove the SRT in about a month so grabbed the 87 off habit. When I left my parents house later that day my car was running like shit and the first thing i thought about was the gas I had put in earlier that day. The good thing was i had only filled it half way so i was able to fill it with 93 to alleviate the issue.