If it's not tuned for it or says to use premium, use 87. Like 05horse said, your wasting your money. Also 91 is harder to ignite than 87, most believe it's the other way around but its not. That's why people with alot higher compression or higher timing uses 91 or 93, to prevent detonation, slow burn rate. Alot of cars that should use 87 but put 91 or 93 in can have driveablitly issues due to it. Alot of time it can't combust all of the gas in the cylinder and it gets pushed out of the exhaust. This can actually harm your catalytic converts if you have them. With todays gas prices, if you can use 87, use it. I have to use 93 in my L and Cobra, the Expedition has a 91 tune but I think i'm going to get a 87 tune for it.
2014 Mustang GT, A6, Grabber Blue