Gas mileage

Possibly. Higher octane fuel does not produce more power or fuel mileage. It allows engines to be TUNED for more power. That usually means a higher compression ratio which can result in spark knock. Higher octane fuel is harder to ignite so spark knock is reduced. A lower octane fuel might cause spark knock which will be detected by a knock sensor, (if your engine uses one). The Engine Computer will respond by retarding ignition timing which reduces power and fuel mileage.

Using a higher octane fuel in an engine that is designed for 87 octane will not gain anything by way of power or fuel mileage.