I am working with engines and will try to clear questions about what gas to use and fuel economy you get, Definitely fuel economy will improve after brake in period (2k to 3k miles), but most critical is what fuel your engine is tuned for! If you look inside your fuel door, it might have two statements; first - "premium fuel only" (edge ST, focus RS, mustang 2,3L turbo), it means that for NA engine tuned for octane rate of 91 (in some states 91 is highest rate you can have) - best performance and fuel economy you will have with 91-93 octane rate fuel and might harm your engine if use 87 fuel: second - "premium fuel recommended", it means that your engine tuned for octane rate of 89 and your best performance and fuel economy you will have at 89 - 91 octane rate, but will not harm engine if use 87 fuel,
