Using 91-octane premium fuel instead of 87-octane regular fuel will only increase your engine's power if your car's manufacturer specifically designed the engine to require it.
Higher octane fuel resists knocking or 'pinging' under compression, which is a premature ignition of the fuel-air mixture. Engines designed for higher octane can take advantage of this resistance to operate at higher compression ratios or with more advanced timing, which can lead to increased horsepower and efficiency. However, if your vehicle's owner's manual specifies regular 87-octane fuel, using premium 91-octane will likely offer no discernible performance benefit and is generally not recommended. Always consult your vehicle's manual for the correct fuel grade.
Fuel octane requirements are determined by the vehicle manufacturer and using a higher octane than recommended typically provides no benefit.
Does premium gas improve gas mileage compared to regular gas?
Premium gas generally does not improve gas mileage compared to regular gas unless your vehicle is specifically designed to require premium fuel. In cars that need premium, using it allows the engine to run at its optimal efficiency, potentially achieving the mileage figures stated by the manufacturer. If your car runs on regular 87 octane, using premium 91 octane will not increase your miles per gallon and will likely result in spending more money for the same or even slightly less mileage due to the higher cost per gallon. The octane rating primarily affects knock resistance, not inherent fuel economy.